Open Emu is an open source project to bring game emulation to OS X as a first class citizen, leveraging modern OS X technologies such as Cocoa, Core Animation and Quartz, and 3rd party libraries like Sparkle for auto-updating. Open Emu is based on a modular architecture, allowing for game-engine plugins, this means Open Emu can support a host of different emulation engines and back-ends while retaining a familiar OS X native front-end.

Currently Open Emu incorporates the following game engines as plugins:

Nestopia
adds support for NES/Famicom
Gambatte
adds support for Game Boy and Game Boy Color
CrabEmu
adds support for Sega Master System, Game Gear, SG - 1000
Visualboy
adds support for Game Boy Advance

Open Emu also supports high quality scaling using Open GL, multithreaded video and audio playback as well as individual features and preferences per game engine, such as NTSC video signal emulation for the Nestopia Core.

http://openemu.sourceforge.net/