Voodoo Vince Jet Set Radio Future Unreal Championship 2 Alien Hominid
Out of these, I think I probably love UC2 the most, as I used to play this with my brother for hours as a kid. Admittedly, I always preferred the Dreamcast to the Xbox, though!
Haven't played NBA Showtime, but I have played stuff like Quake 3 Arena, ChuChu Rocket, Sonic Adventure and Power Stone. If you loved the Xbox, it's well worth picking up a Dreamcast