RetroArch is a cross-platform, sophisticated frontend for the libretro API.

EmulationStation is a graphical and themeable emulator front-end that allows you to access all your favorite games in one place, even without a keyboard.

Multi Emulator Super System (MESS) is an emulator for many game consoles and computer systems.

Higan is an emulator for multiple Nintendo video game consoles, including the Nintendo Entertainment System and the Super NES.

NESBox is a Javascript NES emulator.