Released in June 2005:
http://www.lunaran.com/maps.php?map=lunsp1
This map made by Lunaran is a huge single player map with 137 enemies on skill 1 (normal) and 199 on nightmare. (You could probably DM on it, but you'd never find anyone else).
It was released back in June and really pushes the limits of the look and feel of Quake maps (and kicks some serious ass).
It is a futuristic base style map with Quake 3-like structures and geometry, and does some things with platforms that are cool as hell.
I've seen a couple of smaller deathmatch maps like it, but this map is just mind blowing.
It has enforcers, but also mega-enforcers that throw grenades and use some sort of energy beam and some type of shielded red super-enforcers that can be harmed while firing. (that are spin-offs from imperfect and somewhat awkward ones from an old mod --- but either someone perfected them or the author did, because these ones aren't awkward).
The map itself is has several tiers (upper levels) and getting into some of these places is hard.
I've never seen anything quite like this. It was constructed over a 4 year period.
(The download includes the QuakeC source files for the custom enforcers).