Metropolis street racer.. to be honest the best racer on dreamcast!
Phantasy Star Online - sure the online servers might be down but you will be in awe
Crazy taxi 1 or 2 (but not both) - both are good but basically the same game.. still both classics though!
Space channel 5 - excellent game very nice
Samba amigo - a dance dance revolution game but with marracas!!
Sonic Adventures 2 - the mascot in the better version of the platformer
Ikaruga - i shouldnt say anymore.. coolest game ever
Chu chu rocket - BEST puzzell-em-up on any console
SOUL CALIBER - the god of 3d fighting games on the dc you will not go wrong
Good luck finding all these games! but check out
Dc emulation for some emulation stuff for dreamcast