Team Fortress 2
Recently started getting into Team Fortress 2, all started one night when I was bored and wanted to play something shootery, unfortunately I had already completed Call of Duty 4, so since I got TF2 with the Orange Box, I thought I'd give it a go.
Now I can't stop playing, Soldier being my strongest class, and Spy being my weakest, with the occassional work on the QuickPic application, before my friends I've made on it invite me to play more.
Not as addictive or time consuming as World of Warcraft, where a guild will disown you if you don't turn up to a raid, my steam group don't mind if I don't play. Which gives me tons of time to do other things.
I might sell my WoW account when Wrath of the Lich King is released, there is no way I'm wasting my life on that forsaken game again.
Anyone who hasn't played it yet I recommend they do, it plays differently to most FPS multiplayers. Also get Ninja Reflex on Steam too, the Wii price is a ripoff and if you own a Wiimote, you can make it work as a mouse pointer anyways.