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.
The fork is great, but the main developer is incredibly hostile to contributions (i.e. very, very rude and combative for absolutely no reason). If you are thinking of contributing, keep that in mind - it's not you.
"For code contributions, it's best to start out with modestly sized fixes and features and work your way up. There are so many different possible implementations of more major features – many of which would cause undesirable code or vision conflicts with OpenMW – that those should be talked over in advance with the existing developers before effort is spent on them."