BAR - Beyond All Reason, for Total Annihilation/Supreme Commander style RTS, learned about this one in another thread here on lemmy

Zero-K - similar premise, plays different from BAR, also the graphics seem less demanding

OpenSoldat - 2D arena shooter. For anyone that never played or saw something similar, think of multiplayer maps of halo, quake or unreal, but if it was a 2D platformer

Not quite open source, but Daggerfall Unity is a FOSS update to the engine for Daggerfall, a game which Bethesda has made free for years now. You can get the game off GOG, too.