LaunchBox - LaunchBox is a portable, box-art-based games database and launcher for DOSBox, emulators, arcade cabinets, and PC Games. Download it free!
RetroArch - RetroArch is a frontend for emulators, game engines and media players.
GameEx - GameEx is considered to be the most powerful and feature rich front-end for MAME, GameBase, Daphne, PC Games and all command line based game emulators, along with being a complete Home Theatre PC solution.
mGBA - mGBA is an open source emulator of the Game Boy Advance. The goals are speed and accuracy.
GamersGate - Buy and download games instantly for PC and Mac.
DeSmuME - DeSmuME is a freeware emulator for the NDS roms & Nintendo DS Lite games created by YopYop156.