Off of the top of my head...
jenesisDS (Genesis emulator with sound...given the limits of the DS this is really impressive)
AprenticeMinusDS (a Master System emu by Lordus who made jenesisDS)
snesDS (an earlier SNES emulator, though Snemul is better at this point)
Flubba's Master System emulator...don't quite recall the name but you can find it here or on his site, NDSretro. He's also got some single system arcade emulators.
msxDS
MaracaDS (emulates some older arcade games)
NeoDS (NeoGeo...given the power of the DS it's astoundingly good)
NesterDS (a port of the NES emulator Nester)
Anyway, there are more...you're best off just looking in our DS section.