SNES, Genesis, master system, Atari, NES, Game boy, Game Gear, and recently Game Boy Advance...
All of these things and more were emulated pretty well on the Dreamcast. I don't see how they could not do it to the box. Of course... Dreamcast was a piece o' cake to write homebrew software for. We'll just hope our trusty Enigmah, Xtender or whoever, decide to produce some serious different discs for the system.