Forget the cookie cutter list of games everyone shows when mentioning the Dreamcast.
I highly recommend the following:
Armada: It's like Asteroids, Sinistar and Star Control all wrapped in one unique RPG shooter.
http://www.youtube.com/watch?v=bNe3pDaCDpI
Air Force Delta: I'd thought this game was much better than Ace Combat 2.
Super Magnetic Neo: A unique platform that uses the power of magnetism to advance you character through the levels.
Aerowings: Pilotwings with planes. (I've heard 2 is the better title, but I still haven't played my copy)
D2: Resident Evil style thriller
Star Lancer (Also on PC): If you like Wing Commander or Colony Wars, you'll like Star Lancer.