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.
A square level, with a bunch of fake walls, and hundreds and hundreds and hundreds of rocket packs. Internally, the level is called 'Die', which is relatively accurate - you can't always see what's coming at you, because of the fake walls.
24 levels, some simple, some more complex... all quite playable for small to medium groups. Emphasis is on movement... and it works well. With 24 maps, your net group shouldnt' get bored too quickly...