Unknown Horizons, RTS city builder:
http://www.unknown-horizons.org/

Dungeon Keeper clone:
http://opendungeons.sourceforge.net/

Steel Storm:
http://one.steel-storm.com/

Mario clone:
http://supertux.lethargik.org/

Settlers 2 clone:
https://wl.widelands.org/

Mario Kart clone:
http://supertuxkart.sourceforge.net/