http://www.cosmosui.org/ <- seems to be the best one so far.
http://wow.warcraftstrategy.com/ctmod/ <- another one.
personally i just want a few things out of a UI.
I want the DPS counter, better damage info on the mob, (de)buff time on chars/mob, REAL time clock (so i can see the sun is about to come up and i should go to bed... :)) and better chat window support.
Sure the first two will only help in end game against mad evil raid mob's, but hey i can dream :)