Defense of the Ancients 2. Looks similar to League of Legends or any other MOBA game, although from what I hear it'll have a little more management necessary or some such.
Anyway looks interesting, it'll be free to play once it's out for full public. Matches are available through a buy-in on Steam, but if you DL it for free you can at least watch live games. Lots of interesting looking characters.
http://www.dota2wiki.com/wiki/Heroes