Oh, there is so many.
First of all, IKARUGA. The polarity shooter game that wins too much to not hail treasure every time its name is spoken.
Don't even read any further yet. Get that and play it NOW.
OK, then, the rest:
Cosmic Smash (smash meets breakout, has a really tron-y, tech-y feel)
Rez (Musical Rail Shooter) (same tron-y, tech-y feel, musical shooter where you can really feel the rhythm)
Space Channel 5 part 2 (a continuation of the first, with michael jackson! awesome DC fans have translated the whole thing using the PS2 release of the game, so get that translation to REALLY enjoy it!)
SegaGaGa (you run SEGA in this game. And you meet every old SEGA character. Alex Kidd is a major character. And there's a full space shmup within that has you battle all of sega's hardware, that shoots sprites or polygons relative to said hardware's capability. and you can make parody games with your company. How much more do I need to say before you grab this game? Oh, and be sure to find a translation guide to get you through it.)
Napple Tale: Arisia in dreamland (This is one of the girliest games ever. It's anime alice in wonderland. And it's an AMAZING platformer/RPG. And the music is remarkable. And it's a whole lot of fun. Get it. Also grab a translation guide for this one.)
ZeroGunner 2/Karous/border down/under defeat/trizeal/chaos field/the billions of other DC shmups (these are the normal JP-only SHMUPS that are as many on the dreamcast as there is water in the world. Few are bad, so grab all you can.)
That's all I can pull up for now, happy playing! The DC is an amazing console that will never die, no matter how hard you try. It's the "zombie console"!