You can run Atari ST games, Commodore 64, NES, SNES, Gameboy, Gameboy Advance, NeoGeo, Flash Games and more too numerous to mention. More.
The Wiz can do a host of other useful things besides gaming. It can play video files including MPEG4, Xvid and DIVX. You get a built in e-book reader, calendar, voice recorder... etc... you know the drill. 1GB of storage is built-in and you can expand on this with the SD (SDHC) card slot. The high-res touchscreen makes navigation easy and the gaming controls feature a d-pad on the left with buttons/d-pad on the right and two shoulder buttons on the top. It's plenty good for anything a retro game can dish out. There is an active development community for the GP2X Wiz and with a respectable ARM9 533MHz Processor overclockable to 800Mhz you've got plenty of power to play with if you're interested to write your own applications.