- NES: gpfce. Fullspeed, two players with external joypad
- Gameboy/GB Color: GNUBoy. Fullspeed
- GB Advance: GpSP. Fullspeed in most games if you use the very last version (0.9-2)
- SNES: SquidgeSNES and DrPocketSNES. Although they share most of their code, some games work better on Squidge's and others on DrPocket. Pick whatever or install both and test your games.
- Gamegear and Mastersystem: DrMDx
- Megadrive/Genesis: Picodrive and DrMDx. I think that Picodrive works better and supports two players with external joypad, but DrMDx looks nicer and runs GG/MS as well...
- Neogeo: GnGeo2x (one player) and Mame (slower, but two players)
- CPS2: cps2emu (two players) and Mame (much slower, use only for cps2 games if you really need space in the SD)
- Other arcades: Mame (two players, only choice for most arcades).
Other machines: look up
http://wiki.gp2x.org/wiki/Emulation