For me its COD4, KUF: Circle of Doom, and Rock Band mostly. I still like to jump on Halo 3 to mess around in campaign and forge, too.
Because of the XBL service problems over the last few weeks, we're getting the XBLA game "Undertow" for free this coming week. Its a multiplayer shooter so I'm sure we will be giving it a spin at least for a bit. The price is right anyways!

EDIT: I'd like to play "Catan" with you guys on XBLA, but nobody else in AT has bought it yet. Its lots of fun, but when I play online with random noobs I just end up with stupid 13 year old kids. Download the demo and try the tutorial to get a feel for the game and decide if you want to buy it.