I just bought a dreamcast a month ago so...
Sonic adventure: it is a great game if you don't mind corny cutscenes
Sonic adventure 2: I haven't tried it yet but it looks and sounds better from what I've heard
Dynamite cop: I enjoyed this game, it is an arcade transfer, it's great if you like button mashing fighters.
Gauntlet Legends: I would Highly suggest getting this it is somewhat of a 1-4 player rpg platformer, it is somewhat repetitive, but all in all I love this game, especially for a multiplayer game.
Crazi Taxi: I've never played it, but heard from people here that it is good.
all in all, it is a great Idea to buy the dreamcast, I don't play mine much but we are waiting for the store to get more games in, we only have 2 right now, but in any case you won't be disapointed, the controllers are my favorite ever, because they fit perfectly in your hand and have a little screen memory card, the system itself is great except don't leave it on carpet, or anything like that, and if you buy an original console they will overheat after 5 hours, but who's going to play for 5 hours straight
games on my waiting list:
#1 Gauntlet legends
#2 nhl 2k2
#3 sonic adventure 2
#4 Hydro thunder
#5 coaster works
that's all I can think of off the top of my head.
@ anyone who knows: is half life available for the dreamcast?