Videos of various emulators in action:
Neo-Geo (Alpha Mission, Metal Slug2, Nam'75, KOF'96 etc.)
MegaCD/SegaCD (Dual CPU Enabled) - Rare version of Final Fight
MAME (Bubble Bobble)
Amiga (Xenon II)
SNES (Lemmings, mario etc.)
The GP2X's powerful dual CPU setup makes it ideal to emulate other systems, meaning you can
enjoy many of your games from the past in the palm of your hand.
Systems which the GP2X can emulate include:
Amiga, Atari ST, C64, Spectrum, Amstrad CPC, Megadrive/Genesis, Mega-CD/Sega-CD, SNES, GB, GBA,
Neo-Geo, MAME, NES, SMS...
The good news is that the emulators are very mature, easy to use, fast and easy to install.
For a more indepth look at some of the most popular emulators see Craig's review.