I looked through the first 5 pages for a thread like this and didn't find one so here it goes...
League of Legends - Awesome MOBA and completely free to play
Dirt 2 - Fun time killer, love the graphics and the cars, wish there were like 2x as many courses though
Heroes of Newerth - I don't play this as much as LoL since I just got in to beta a couple weeks ago but the few games I've played so far have been awesome. <3 Andromeda
Starcraft 2 - Played this a shit ton when I first got in a month ago or so, now I play just a couple times a week because I'm pretty awful =/
Bloodline Champions - a battle arena game, think WoW arena sort of. Still in beta and servers are only up on weekends so I don't play that often but it is pretty cool.
...and as soon as I post this I see there is the same thread on page 1 that I missed >< Mod could delete this since I can't or just move this in to the other thread