RetroArch - RetroArch is a frontend for emulators, game engines and media players.
Lutris - Lutris is an open source gaming platform for GNU/Linux.
Mednafen - Mednafen is a portable, utilizing OpenGL and SDL, argument(command-line)-driven multi-system...
mGBA - mGBA is an open source emulator of the Game Boy Advance. The goals are speed and accuracy.
OpenEmu - Console and Arcade video game emulator for macOS.
GOG Galaxy - Truly gamer friendly, DRM-free online gaming platform.