SegaXtreme

Rough but playable versions of the following arcade emulators have been released for the Dreamcast:

XMen

Teenage Mutant Ninja Turtles (updated today)

The Simpsons

Cadillacs & Dinosaurs (updated today)

Vendetta

Wonderboy

Street Fighter II Alpha (it's not Street Fighter Alpha 2)

Contra

a few more too

I'll post more info in this thread for individual games, but you can check out DCEmulation and their message boards at the following links in the meantime

http://www.dcemulation.com/

http://www.dcemulation.com/phpBB/