Snes9x is a Super Nintendo emulator for a variety of platforms. It works under Windows, Mac OS X and Linux, among others. If my memory isn't failing me, Snes9x was first released for Windows, hence the last two letters of the name. Its Mac version works very well, and it has a whole array of features.
The most important thing about an emulator is its ability to run games properly. Snes9x worked fine with the few ROMs that I tested it with. The game played at normal speed and sound was identical to the original game. The controls are intuitive and are the same that most other emulators use. The arrow keys are the D-pad and control and command are the two main buttons. You can change this from the settings. By default, Snes9x runs in a small window, but you can enable the full-screen mode from the preferences window. You can't switch between full-screen mode and windowed mode in real time, though. With the left click you pause emulation and with the right one, you resume it.
Some of the advanced features allow you to record videos of gameplay, use cheats, and save the games and resume them later.
The downloads page of Snes9x seems to be down. But if you access the News section, you can access the official mirror and download version 1.5.2, the latest version for the Mac.