Playnite - Source code generated using layoutit.com
GOG Galaxy - Truly gamer friendly, DRM-free online gaming platform.
Lutris - Lutris is an open source gaming platform for GNU/Linux.
Heroic Games Launcher - Heroic is an Open Source Game Launcher for Linux, Windows (Beta) and MacOS (Limited to Windows games using Wine/Crossover). Right now it supports launching games from the Epic Games Store using Legendary, a CLI alternative to launch epic games.
LaunchBox - LaunchBox is a portable, box-art-based games database and launcher for DOSBox, emulators, arcade cabinets, and PC Games. Download it free!
itch.io - An online game marketplace and community.