At the moment I don't have Live at my house so I am only on it once every other week or whenever I can drag my card to a gaming center, but about halfway through January 2008 I will be on pretty much constantly.
Online, I like to play Halo 3, Puzzle Fighter, Uno, SF2HF, UMK3, Guitar Hero III and probably in the near future Team Fortress 2.
Offline, my favorite games for 360 are Conan and Overlord.
Feel free to add me, my list is currently pretty small and I could always use more.
I am especially looking to get in more GH3 co-op, plus when I am on Live I will pretty much be 'game' to play anything I have/can, even if it isn't "new" or "popular."