ZSNES

ZSNES is a great SNES emulator. It supports OpenGL for output, along with many different filtering methods to enhance the visual experience. It also has stereo sound, and even has an option for surround for you avid Tetris players.

In addition to standard emulation features such as snapshots and gamepad support, ZSNES supports speedup and slowdown buttons, key combinations, replay (jumping back some seconds in the game to redo a part), cheat code search (to make your own Game Genie-like cheats) and network play.

It is written in x86 assembler and is therefore pretty much unportable. Snes9X works on other architectures.