RetroArch - RetroArch is a frontend for emulators, game engines and media players.
Ubisoft Club - Ubisoft Club app enables you to get updates about your favorite games timely.
Lutris - Lutris is an open source gaming platform for GNU/Linux.
itch.io - An online game marketplace and community.
mGBA - mGBA is an open source emulator of the Game Boy Advance. The goals are speed and accuracy.
GOG.com - DRM-free game store, selling both new and old titles. No clients required.