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 was originally created by Brad "Carnevil" Carney, then transferred to a development team as Carney tackled new projects. The current version (0.98d) is based on ZDoom 2.3.1 revision 1551 and GZDoom revision 127/323.
Skulltag went open source on the 13th of February 2012. On the 4th of June 2012, it was discontinued, when the development team forked the project into Zandronum to acquire autonomy from Carney.