Genesis Plus is a free Sega Genesis emulator for the Mac. It is an incredibly light and polished application that lets you relive those fun moments playing the Genesis, but on a current Mac. The first thing that I would like to say about this emulator is that it works like a charm. This developer really can produce great-quality apps and one of his trademarks is that all of his emulators are very accurate and quite customizable. To play a game with this emulator, you will need a ROM file. There are some legally free ROM games available, and you can dump your own ROM files if you own the game. I tested this app with the ROM image of Tecmo World Cup, a soccer game from 1993. It was my favorite growing up and I still enjoy it. Many people have favorite games from back then, and chances are that it is available for the Genesis, and that Genesis Plus can play them quite well.
When you run Genesis Plus, the app will ask you for a ROM file, and after you load it, the game will start playing. The default size of the screen is quite small, and believe it or not, that is the resolution the games ran at on the Genesis. You can drag and drop on the corners to change the size of the window. I enlarged it to half my screen, and the graphics still looked good. The emulator supports joypads and you can also control the games with the keyboard. You can customize the controls for either device. There are also some graphical and emulation settings that you can change from the Preferences window. These include emulation speed, sounds volume, frame rate, effects size, etc.
In short, Genesis Plus is a great emulator. It works well with all of the games that I tried to open and it is very stable.