OpenEmu may just be the best emulator available for the Mac. This awesome emulator supports an array of different gaming systems that it'll be nearly impossible for you to get bored.
The biggest problem with most emulators is that they're ugly and very clunky in use - they're not aimed very well at the casual gamer/user. OpenEmu changes that completely.
OpenEmu sports a fantastic UI that is very simple to use and is actually inviting when it comes to playing games.
Currently OpenEmu supports a whopping 12 different gaming systems including - Game Boy, Game Boy Advance, SNES, Nintendo DS and Sega. Notable ones such as the Nintendo 64 and PlayStation are missing, but the developers have said support will be coming.
Why put up with a keyboard and mouse when you can play with a controller. OpenEmu supports an array of different controllers but probably the easiest to hook up to a Mac is a Dualshock 3 or 4. They connect over Bluetooth and are very easy to setup. OpenEmu will then let you map out controls to the controller so they're spot on.