I have play-moods where I eventually get bored of a game and get a hankering to play another one. I used to play Bioshock for over a month, then I'd get bored of it and start playing Zoo Tycoon instead. When I get bored of that I began Half-Life 2, or Jedi Knight: Jedi Acadamy. Then I eventually get around to replaying Bioshock again. The cycle goes on.
Currently though, I'm playing Fallout 3 (with assorted add-ons) and Team Fortress 2. I may get in a mood to play something else inbetween, but for the time being, those two are fine.