Skulltag
Skulltag is a source port based off ZDoom originally designed with improved multiplayer as its primary goal, but it has since expanded to include a single player mode, complete with new monsters. It is maintained by Brad "Carnevil" Carney. The current version (0.98d) is based on ZDoom 2.3.1 revision 1551 and GZDoom revision 127/323.
The OpenGL renderer in Skulltag is the one from GZDoom 1.1.04 revision 323. Because of a license change that happened in r128, the GZDoom-specific codebase will not evolve further. However, Torr Samaho negotiated an agreement to port maintenance changes which are needed to keep up with ZDoom codebase changes or to fix bugs.
Though Skulltag is closed source, the source code to an older version (97rc2) has been released.
