PocketMVP if you want to watch Divx video (you'll likely have to reencode to be smaller), PocketNester is the best Nintendo emulator, PocketVCS if you like old skool (Atari 2600 emulator), LEDHead if you want to go really old skool (old Mattel handheld emulator). Those are all freeware, and there are a ton of good pay programs and games.