RetroArch - RetroArch is a frontend for emulators, game engines and media players.
Lutris - Lutris is an open source gaming platform for GNU/Linux.
Playnite - Source code generated using layoutit.com
mGBA - mGBA is an open source emulator of the Game Boy Advance. The goals are speed and accuracy.
Batocera.linux - Batocera.linux is an open-source and completely free retro-gaming distribution that can be copied to a USB stick or an SD card with the aim of turning any computer/nano computer into a gaming console during a game or permanently.
GOG Galaxy - Truly gamer friendly, DRM-free online gaming platform.