There's also a fork with multiplayer support (to an extent): https://github.com/TES3MP/openmw-tes3mp OpenMW is great, played it a lot a couple of years ago. I love when games get future-proofed like this. See also the various Quake and DOOM source ports, although those started from id Software releasing their engines under the GPL so had an easier start.
granted there are some limitations you have to be willing to overlook to enjoy the game .. mainly around questing and syncing since all players share the same quest ledger. for example (SPOILER) when curing myself of corpus my character received the cure and attribute buffs but my teammates did not. in the main story line these sync issues are infrequent and were manageable, but at times got especially bad in the DLC -- be prepared to hop in to the command line a lot to revive quest givers, update the journal, acquire items, alter stats.