Open Source Games
An open source game outlives the studio that made it - no server gets switched off, no store revokes your copy, and the mod community keeps adding to it for decades after a commercial title would have been delisted and forgotten. The games here ship with their source and assets open, so a classic stays playable on hardware that didn't exist when it launched, the community can fix and extend it freely, and the worlds you sink hundreds of hours into can't be taken offline out from under you.
Mindustry
Automation tower defense RTS with sandbox building and multiplayer play
OpenRA
Libre real-time strategy game engine for classic Westwood games, with modernized play and mod support
Luanti
Open source voxel game engine for easy modding and game creation
OpenTTD
Open source transport simulation game with multiplayer, add-ons, and cross-platform support
Endless Sky
Open source space trading and combat game with exploration, missions, and ship upgrades
Veloren
Open source voxel RPG with multiplayer, procedural terrain, and official builds for desktop platforms
Battle for Wesnoth
Open source turn-based tactical strategy game with campaigns, multiplayer, and a map editor
OpenMW
Open-source engine reimplementation that runs your copy of Morrowind on modern systems
SuperTuxKart
Free kart racing game for fun-focused play on desktop and Android