• 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/