OpenEmu is a Cocoa framework and application for running multiple emulators as plugins. Many popular open source emulators; such as SNES9x, Nestopia, and VisualBoyAdvance; are fully compatible with this system. As all the emulators are going through the same architecture, it can be tuned to use the latest Quartz and Core Graphics technologies to deliver screaming performance.
Most of my participation has been in fixing bugs and doing some application-side coding. My big task so far is an in-development ROM organizer powered by Core Data, with smart playlists, Quick Look for save states, and ratings.