DIII Mage Tank!
So here's the build I've been using for my Mage Tank. I'm really liking it. I've been playing with replacing Cold Snap with Bone Chill for some more damage. The 3rd passive is pretty much up for discussion too - if I had a lightning/fire/cold weapon I'd be using Paralysis/Conflagration/Cold Blooded. I run with the enchantress follower for the armor and attack speed buffs.
I've been using her for solo play, but I'm told the real strength of the build is in groups where you have some some real glass cannons backing you up. (For group runs, swap out the venom hydra with Slow Time & Time Warp.)
The general idea is that you can almost keep stuff perpetually stun-locked using frost nova in combination with critical mass. Energy Twister with Wicked Winds crits almost constantly so that your cool down on Frost Nova and Diamon Skin are almost none existant (Meteor with Star Pact works OK with more damage, but harder to control). You pretty much just run face first into packs of mobs, even elites, and pop twisters right on top of them while contstantly refreshing frost nova and diamond skin until everything is dead. It's a low dps build that relies on high resists/armor along with a good amount of life on hit, and a near constant stun lock to slowly burn through stuff.
With only about 200k spent on gear I had been having trouble with Act II Inferno, so I dropped back to Act I and it was easy farming. I found an amulet that sold for 2M so now that I have some decent gear Act II is pretty easy going. Next time I get decent internet connectivity I should be able to finish up act II and see how far into act III I make it.
Here's the threads this build is based around if anyone wants the long read.