MechAssualt is pretty fun. Also Ghost Recon and Jedi Outcast. There even better multiplayer.
So far for XBox I own...
Ghost Recon
Jedi Outcast
Mech Assualt
Splinter Cell
Jet Set Radio
Sega GT 2002

I plan on getting Halo as soon as I can find it, and Tao Feng in march, and Star Wars Knights of the old Republic this month.