- Beyond All Reason (Supreme Commander, Total Annihilation clone)
- Warzone 2100
- Liero
- Hedgewars (Worms clone)
- OpenTTD (especially JGRPP fork)
- GZDoom / Woof / DSDA and other DOOM 1 and 2 source ports
And some that require original assets:
- OpenMW (The Elder Scroll: Morrowind engine replacement)
- OpenXcom (especially OXCE and Brutal-OXCE forks, they all have great mods, like XCom Files, Harmony, XPiratez)
Also, check this open source or source available games list: https://osgameclones.com/