iOS: Joypad, a free application for iPhones, turns your phone into a simple NES game controller–additional premium skins replicate other classic controllers.
Joypad turns the touch screen of your iPhone into a game controller. Included in the basic free app is a simple NES style directional pad and A/B button system. You can, for $0.99, get additional skins and layouts for SNES, N64, and other controllers. Joypad is supported by both Macs and PCs via both Wi-Fi and ad-hoc connections.
Check out the video above to see someone playing Super Mario Bros. in emulation using JoyPad to control the action.