OpenEmu is a ROM library management and emulator front-end application. That’s what enabled OpenEmu to come along and put a front-end on the emulation cores of a dozen or so different emulators. Thanks to the authors of those emulators, much of their work is open-source at this point too. Basically any living room console older than the year 2000, and all handhelds before the current generation (before 2011 or so). The lower-powered game consoles have all been well emulated by this point.
