http://www.curse-gaming.com  While they have the largest database off 2.0 mods, they also have the highest traffic rate.  Goes up and down all day long, so if you get it, find something good quick.

http://www.wowinterface.com Sexiest site out there. These guys support the Ace2 formats, which generally are smaller and take up alot less memory space to run. They have a low bandwidth site up, and thier regular site is good to go now too.

http://ui.worldofwar.net This one rivals curse for size, but doesnt catch quite as many updates. Warning, VERY slow site, least right now.

http://www.wowace.com/files. This one tells you how hardcore you are. Simple file list of all mods that are hosted.They just now put the full site back up, so hopefully you can root around and find anything you like.

http://www.insomniax.net Since I wouldnt have a UI without this, Ill post it here.  One of the first packs to be updated for 2.0 this team edits ALOT of the existing mods with extra functionality.  Some people use the whole thing, but you can simply pick and choose what you like.