Cataclysm Dark Days Ahead has been mentioned so I’ll add OpenMW to the thread. It’s a open source engine rewrite of Morrowind and requires the base game to run Morrowind but it has the potential to be used to create new games and the devs are looking to expand it to allow the creation of ARPGs as well.

Another worthy mention is Endless Sky, it’s an open source game inspired by the classic space trading and combat game Escape Velocity.