Wikipedia
OpenEmu is an open source multi-system game emulator designed for OS X. It provides a plugin interface to emulate numerous consoles' hardware, such as the Nintendo Entertainment System, Genesis, Game Boy, and many more. The architecture allows for other developers to add new cores to the base system without the need to account for specific OS X APIs.
Version 1.0 was released on December 23, 2013, after a lengthy beta testing period. Numerous incremental updates have been released since then, with plans to incorporate support for more consoles in future releases. Some of these in-development cores are available to download in an optional "experimental" cores build (released alongside the regular, "standard" version), containing support for such consoles as the MAME and Sega Saturn.