Good Open Source Games
FPS
Xonotic: Fast-paced arena shoter
Sauerbraten: fast-paced arena shoter with in-game map editor
Survival
Cataclysm Dark Days Ahead: Insanely in-depth turn-based survival game
Minetest: Easily modable MC-like voxel game
Strategy
Beyond All Reason: RTS game simular to Total Annihilation
Battle for Wesnoth: turn-based strategy with a high fantasy
Unciv: Inspried by the civilizations games but with a minimal graphics style
Mindustry: Sandbox tower-defense game
OpenTTD: Transport company simulation game similar to Transport Tycoon Deluxe
Space
Endless Sky: 2D space trading and combat game similar to Escape Velocity
Pioneer: A 3d space trading/exploration/combat game similar to later Elite games
Oolite: A lot like orginal Elite but with full 3d graphics
Car/Racing
Rigs of Rods: Soft-body vehicle simulator with lots of community made maps and vehicles
SuperTuxKart: 3D arcade kart racer
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