OpenTTD is a re-implementation of TTD, though. OpenKSP would be the equivalent (which doesn't exist).
Quake 3 Arena is a better example, which has open source code but proprietary graphics. OpenArena is the open variant.
Or HoverRace, where the developer released the source code but extremely late. The game was abandoned around 2000, community dead by 2004ish, and code released a few years after that. Too late to really save it, but it was still nice to play now and then.
Warzone2100 would be a proper example of an commercial game having a open source afterlife (including being ported to linux/mac and continued development to this very day).
Quake 3 Arena is a better example, which has open source code but proprietary graphics. OpenArena is the open variant.
Or HoverRace, where the developer released the source code but extremely late. The game was abandoned around 2000, community dead by 2004ish, and code released a few years after that. Too late to really save it, but it was still nice to play now and then.