As a cleric tank there are many specs that allow you to tank. Here is a overview of a lot of specs with their advantages and disadvantages.

Basic needs

38 Justicar / 7 Shaman

This is the spec that has all the mandatory talents though a few points can be switched around. All talents builds should have at least all these talents picked up (healer's greed can be switched to Doctrine of Bliss or Hammer of Virtue).
The remaining 21talent points can be changed nearly everywhere to pick up the thing you want. Below are a few of the more common examples.

51 point builds

The following builds will all have 51 points in justicar which will give you DoA which can be very helpful experts with a healer that is a bit weak/undergeared because you can assist party healing by a lot. The also give you a lot of armor because MoL gives 1% additional armor per point spent in the justicar tree.

Basic with Inquisitor: 51 Justicar / 9 Shaman / 6 Inquisitor
This one is nearly similar to the spec above however it has changed to 5% spellpower in sentinal for 5% spell crit and 50 endurance plus 215 armor from Armor of Treachery. With this change you do lose courage of the bear but if you got someone else with a strength aura this is no problem as they don't stack (and you can still use courage of the Jaguar to keep 40 dodge). Also notice that because you lose 5% spellpower you will generate slightly less threat however you can switch to armor of Devotion to get 42SP and SC which can help a bit. 

Basic with Sentinal: 51 Justicar / 10 Shaman / 5 Sentinal
This is one of the most basic tanking builds. It has all the points in justicar which gives you Doctrine of Authority. It has all the mandatory talents in shaman and a few more to pick up Courage of the Bear that will increase your block by a additional 40 if you don't have a archon in your party/raid. The 5% spellpower in Sentinel gives you a bit more block, parry and dodge and will also slightly increase your damage and healing (and of course threat as a result). You also get Healing Breath which heals for a little bit more then DoB even with the 40% reduction and doesn't consume a conviction.

Basic with Purifier: 51 Justicar / 8 Shaman / 7 Purifier
In this case 7 points are spend in purifier to pick up 5% wisdom, 6% armor and a additional shield. However the 5% wisdom from purifier gives less avoidance then the 5% spellpower from sentinal or Cabalist. Also a thing to note about the 6% armor is that it's based on your base armor before MoL and talents, the result is that it only gives about 200-300 armor depending on your gear. The shield absorb 831 damage which isn't that much but it's more then a DoB or healing breath from sentinel and can be used before a pull to save a gcd or used when at full health but not taking damage. This is the spec that will give you the highest amount of armor that is possible but with the resent change to Armor of Treachery in the Inquisitor tree this difference is now minimal.

Basic with Cabalist: 51 Justicar / 9 Shaman / 6 Cabalist
This spec also gives the 5% spellpower that the basic spec provides with sentinal. However by spending 6points in cabalist you can pick up Dark Harbor which will increase your elemental (water/fire/earth/air) resistance by 40 and stacks with the resistance provided by a bard or archon. This can be helpful when tanking magic heavy bosses like Cyclorax or Gatekeeper Kaleida and with Hammerknell on our doorstep we can also expect bosses with water damage there. However since it does not provide life and death resistance you won't benefit from it in GSB or RoS or bosses that deal death damage like Gregori Krezlav in DSM. With this spec you also get Obliterate which can be useful for aoe pulls if you build the Lurking Decay on the last mob of the pack before it.
If there are some bosses in hammerknell with water damage this will be the spec of choice for it.

44 point builds

These build change a few points from justicar to pick up different thing in other trees. However they lose some armor and DoA as a result but keep the new AoE taunt Rebuke. (There are a few variations possible with the 44 points in justicar where you drop different points. However all possible combinations will drop Humility. And the remaining points can mainly be removes from DoB, Hammer of Virtue and Supremacy but you can also remove some from Light Makes Right, Commitment or Healer's Creed).

Deeper Shaman: 44 Justicar / 22 Shaman / 0 Purifier/Sentinal/Templer
In this spec you take a few more points in Shaman. This will give you a 360 cleave that can be usefull at the pull when mobs are still around you. However it does less damage then Even Justice and does not give a conviction. You also get Jolt which is off the gcd and can be used when you crit the damage isn't that high because of the 40% reduction but that is countered with the 50% increase in air damage. You also get a second mana cooldown but that's normally not needed but can be useful. You also get Glory of the chosen which will give you a small heal off the gcd whenever you are crit (remember crit immune does not exist in Rift).
By leaving the last soul empty it's possible to switch the soul. This gives you the option to use Sentinel for the instant heal, Purifier for the self shield or even Templar for Break Free (you can break out of crystals at Majolic with it and a few other things). And again there are a few variations possible in the shaman tree.

Purge: 44 Justicar / 12 Shaman / 10 inquisitor
The reason this spec is popular is because it gives you 50endurance+215armor from Armor of Treachery and gives you purge which can be very useful on a lot of boss fights in experts (However since patch 1.2 more people now have purge making this less useful). Some examples are Groniks reflect shield, Warden Falidor or the heal in the Geldo council. It will also give you the small heal from shaman that is off the gcd and can be used when you are crit. There are a few variation on this spec possible where you move some points from shaman to justicar.
Another use for this spec is against Cyclorax where you can actually range tank the boss by using the inquisitor abilty's (remember all life ability's give conviction and all ability's get the +300% threat). This will save you a lot of running around from the aoe and the threat is high enough to not lose aggro.

Deeper Purifier: 44 Justicar / 10 Shaman / 12 Purifier
This spec takes more points in purifier to increase the amount absorbed by the self shield from 831 to 1246. This spec also gets to ability to cure diseases, poisons and curses. This can be very helpful on yourself as it will save your healer a gcd every time. And again you get the 6% armor from Flame Ward that will compensate a bit for the armor loss from the points moved our of justicar (you only lose a 1% increase of base armor compared to 51 points with the exception of the purifier and Inquisitor version).
A nice bonus is that you get a res which will save you from having to switch role (and possible rebuffing) every time you have to res someone.

Deeper Cabalist: 44 Justicar / 12 Shaman / 10 Cabalist
This is a small additional to the 51 build with cabalist. What you get is a bit more damage on your spells (not your melee attacks) and you get a additional aoe damage spell that can be used during those harder aoe pulls by stacking up Lurking Decay on the last mob of the pack before it and using tyranny. But if you don't have any you can use Sigil of power aswell. Other then that it has the same advantages as the basic with cabalist spec.

38 point builds

These talents builds give up a decent amount of armor, Rebuke and DoA to pick up ability's deeper into certain trees. With the change to the justicar tree in patch 1.3 I wouldn't advice a 38 point build anymore as have to give up a decent amount of stuff (In most of these builds a lot of variations are possible so if you want to switch something around feel free to do so)

Deep purifier: 38 Justicar / 7 Shaman / 21 Purifier
This is a build that will give you a lot of Oh **** buttons (It's also one I used pretty often in raids before patch 1.3). The most important ones are Searing Transfusion that will increase your health by 25% and also heal you for that amount (unaffected by the 40% reduction of MoL), a 1246 Shield of Ancestors, Latent Blaze and a guaranteed critical Reprieve with the use of Flashover.
One thing to note about Latent Blaze is that if you buff it before you use MoL it will not be effected by the 40% reduction and you will get the full benefit from it. This means that applying it before the fight is very useful but during the fight it becomes a bit worse (but still worth it). (This also works with Healing Blessing)
The ability Flashover guarantees your next heal will be a critacal hit and can be used with reprieve to always make it a crit which will increase it's effect by 50%. But because Flashover only has a 45sec cd it can also be used for DoL during those heavy AoE phases.
You also keep the dispell that the 44 purifier spec also had and you get a shield that can be used on others which can be handy for those trigger happy dpsers or that low health player during aoe.
One ability I haven't mentioned yet is healing Blessing. This ability heals you for a low amount everytime you are hit up to 6 times. However because it's effected by MoL it only heals a little over 100 per hit. But since it's after a hit it will never overheal and with 6 stacks it will heal a bit over 650 total depending on gear which is not bad for 1 gcd as it's more then DoB of Healing Breath. (and you can apply it before a pull like the shield)

Deep Sentinel: 38 Justicar / 7 Shaman / 21 Sentinel
This goes deep into the Sentinal tree to increase the AoE healing done, pick up Protect the Flock and get Healer's Covenant.
First something About Protect the Flock. It will proc of Reparation and as a result put it on 5 people nearly every gcd. However it will not proc on yourself as Salvation is single target. But if you use DoL it will proc on yourself but I would not advice using it every 6 sec just to keep it up.
A note about Healers's Covenant: It is on a 2min cd but only gives 1 min debuff, as a result it's possible for you and your healer to alternate the ability on you when the debuff runs out. However remember to not use it when you feel like it. Most of the time the healer will know when it's more useful on you because you might get low and pop it but your healer was a micro second away from that big heal that would top you again. Or he might want to use it on you so he got time to do some party heals but he can't because you already used it before and got the debuff. So just remember to speak with your healer about it's usage (unless you got a healer without it of course).
Another advantage of this spec is that you are able to pick up a dispell like the purifier specs. You also get the AoE dispell which is even better then the normal one as you won't have to target anything. You also get a silence which can be very very handy on large packs with casters that you want to group up to AoE down.
(this is also a spec where a lot of changes can be made in the deep tree for example you can take the 20% crit damage and healing instead of the additional heals on healing breath)

Deep Shaman: 38 Justicar / 28 Shaman / 0 purifier/sentinal
The reason this spec is popular is the ability to get a charge and the 360cleave. This makes picking up large packs of mobs a bit easier. But other then that the spec doesn't give much more then the 44 version of the shaman build and you lose your Rebuke to get it.
Again the last soul is free of points to make it changeable as with the 44 point build.
With this build taking points out of commitment seems a pretty logical option because the amount of mana you will have with 2 mana cd's and the mana return on crit will be huge so the mana on block will be unneeded.

Deep Cabalist: 38 Justicar / 7 Shaman / 21 Cabalist
This spec gives you a lot of new additions to the deeper cabalist spec and as a result this build has probably the highest possible aoe threat and a few tricks.
The most important thing you get from taking this build is the ability called Mealstrom. This pulls up to 3 mobs within 15 meters of you towards you which makes runaway mobs easier to pick back up again. However the range is pretty big so be careful not to pull the next pack towards you.
It also has a aoe damage aura every min for 20sec that will also help you keep that aoe threat. And there is are even more like Curse of Solitude and the shorter cd Obliterate.
This is also one of the specs that will have no problem ranged tanking certain bosses.

Deep Warden: 38 Justicar / 8 Shaman / 20 Warden
This is a bit of a unconventional build but can be very useful if you got a healer that isn't that good and will have you from having to take a support along just to keep you alive.
This build will give you a pretty large amount of self healing compared to other builds and your loyalty will also heal for 15% more.
The ability to stack up soothing stream on yourself will soften those big spikes and the healing done will give you enough threat to use a gcd to keep the stack running without losing aggro. You also get the ability called Orbs of the Stream that can be used on yourself as a small defensive cd. And like Latent blaze it can be buffed before MoL which will remove the 40% reduction from it. And in most of the fights there are a few seconds where you can actually reapply it but with the 40% reduction (which is still useful).
This spec also gives you Tidel Surge which increase your next heal by 50% and can be used for Reprieve and Loyalty to make them more powerful.
Also like the purifier and sentinal builds you will have a dispell which saves your healer that gcd.
And last but not least you get a 3sec silence in the form of Drown.

Deep Druid: 38 Justicar / 20 Druid / 8 Shaman
This build like the deep shaman build will generate a larger amount of single target threat. The reason for this is having both Vengeance of the winter storm and Crag Hammer up for more damage every swing (yes they stack). This spec also gives you a ability called Eruption of Life that does a decent amount of damage and some additional damage the next 3 melee hits. This is by far your highest dps and as a result threat spell you have in this build and it generates 1 conviction aswell (yes it does more damage then PoR/Sovereignty). You also get a spell called Bombard that does more damage then SoJ but does not generate a conviction but using it will increase your threat if you don't need more conviction. This spec also gives you a 10sec sleep (30sec cd) and a 5sec silence (1min cd) which can help to make your pulls easier.
And last you also get a additional shield which unfortunately only absorbs physical damage and a ability called Trickster Spirit that will decrease your damage taken by a small amount (does less damage then SoJ so be careful not to lose aggro).
And like the deep shaman spec this also has a second 1min mana cd and makes it possible to take points out of commitment and move them around. (for example to Hammer of Virtue)

Unconventional builds

Under this part are specs that are special and will mostly be useful for a special task. At this time there will be only 1 spec but I might add more.

The healing Offtank: 38 justicar / 18 Sentinal / 10 Warden
This build drops the shaman tree to pick up more healing. The result is that your tanking capability's have drops because you lose 10% healing taken and 3% less damage taken and as a result you will be a offtank (Hylas is one of those fights you use an offtank). However by sacrificing this you severely increase your healing cabality's. For example when not tanking during the hylas encounter there is a large amount of aoe damage going on where this healing can come in handy. You can then which to a 2H weapon (you can switch weapons in combat) and use Mien of Honor. The result is that your AoE healing output is now actually higher than chloromancers or any senticar/shamacar/inquisicar build. The reason is because none these builds are able to pick up DoL, the healing increase talents in Sentinal and the 15% increased healing in Warden.
And you also get Tidal surge which can increase the healing of one of your DoL's by a additional 50% every 45 sec.
Just remember to switch back to a mace+shield and MoL before you have to pick up mobs again (or before the polymorph on hylas) so you can be healed up from the heal increase of MoL.