open-source-game

OpenTTD is a transport simulation game based upon the popular game Transport Tycoon Deluxe, written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.

Unvanquished is a free and open source FPS/RTS hybrid game powered by the Daemon game engine. Historically based on Tremulous as a continuation with decades of work over it.