ExtraMAME is a GUI wrapper for M.A.M.E., the Multiple Arcade Machine Emulator that allows you to play thousands of old-school arcade games on your computer.
MAME (Multiple Arcade Machine Emulator) was designed to allow you to enjoy old arcade games on your current machine. Also available as front ends: MAMEUI, IV/Play and Emu Loader.
xemu is an Open Source application that emulates the hardware of the first Xbox, resulting in better compatibility with kernels, titles, and homebrew apps.
QuickPlay is a universal emulator frontend-of-frontends, which has support for many emulators and systems, old and new, with a philosophy of being quick and easy for new users but powerful for old-school gamers.
Emu Loader is an Open Source GUI for popular game emulators including MAME, Supermodel - Sega Model 3, Daphne, Demul, HBMAME, DICE, Sega Model 2 and ZiNc.