Mednafen

Mednafen is a portable, utilizing OpenGL and SDL, argument(command-line)-driven multi-system emulator with many advanced features. The Atari Lynx, GameBoy (Color), GameBoy Advance, NES, PC Engine(TurboGrafx 16), SuperGrafx, Neo Geo Pocket (Color), PC-FX, and WonderSwan (Color) are emulated. Mednafen has the ability to remap hotkey functions and virtual system inputs to a keyboard, a joystick, or both simultaneously. Save states are supported, as is real-time game rewinding. Screen snapshots may be taken at the press of a button, and are saved in the popular PNG file format.

Due to the threaded model of emulation used in Mednafen, and limitations of SDL, a joystick is preferred over a keyboard to play games, as the joystick will have slightly less latency, although the latency differences may not be perceptible to most people.