The Sega Master System/Gamegear/Genesis emulator for OS X
Kega Fusion is a great multi-platform Sega emulator.
Downloading and Installing Kega Fusion
To download the latest version of Kega Fusion, head over to the Kega Fusion homepage:
Once Kega Fusion is downloaded, move it to your Applications folder, and you're ready to play Sega games on your Mac!