RetroArch is a graphical frontend for emulators, game engines, and media players.
Games library manager. Supports: Steam, GOG, Humble Bundle, Humble Trove, Wine and RetroArch.
Open Emu is an open source project to bring game emulation to OS X as a first class citizen, leveraging modern OS X technologies such as Cocoa, Core Animation and Quartz, and 3rd party libraries like Sparkle for auto-updating.
Mednafen is a portable, utilizing OpenGL and SDL, argument(command-line)-driven multi-system emulator with many advanced features. The Atari Lynx, GameBoy (Color), GameBoy Advance, NES, PC Engine (TurboGrafx 16), SuperGrafx, Neo Geo Pocket (Color), PC-FX, and WonderSwan (Color)...
BizHawk is a multi-system emulator designed predominantly around the production of Tool Assisted Speedruns (TAS). BizHawk focuses on core accuracy and power user tools while still being an easy-to-use emulator for casual gaming.
RetriX is an emulator front end for UWP, on all the hardware platforms it supports: it serves the same purpose as RetroArch but is built from the ground up to integrate as well as possible with Windows.