A couple of years ago, ydnar offloaded the last of his Marathon detritus, a very impressive (though slightly unfinished) map called Last Survivor of the Nostromo. Recently, oogaBooga decided to polish it up.
A 7 level nethop pack. Every single one of these maps is worth playing on its own (well, okay, the vacuum level's sort of a pain in the ass), together they total a monstrous netgame potential.