In my opinion, the greatest map ever designed for any multi-player FPS - if not the greatest, at least the most fun one.
The Well was designed for Team Fortress by Sgt. Thundercok, also known as Jim Kaufman. The last version for TF was "well6", for TF 2.14.
Since then, the map has been redesigned for Team Fortress Classic and Quake III Fortress (Q3F). I have heard nothing of the TFC version, so I don't know if it's any good (the rare times when I played Half-Life last, it wasn't on any of the servers...).
The Q3F version was done quite faithfully, based on the same plan, with blessings from the original author. It looks nifty, except for strange-looking elevators that look quite futuristic, but otherwise it appears to be the same and just nice. (In Q3F 2.2, the map appears in the abbreviated list as "Oh Well"... =)
The level has two fortresses for two teams (not quite unlike the famed 2fort design), and two moats with bridges and a low building separating them. The "bunker" has doors and long wall in the middle, so you need to go through the corridor to get through it.
The level is based on Capture the Flag game (as usual), and the flags are kept in intricate towers with many ways in and out (like the water-filled pipes that require some assistance from Demolitions personnel to use). By the way, there's no "well" in the level any more, but the author didn't bother to change the name =)
Lots of nice sniperance places, but also a lot of fun for each of the character class!
Now, if I'd ever see a remake of the most impressive-looking map, Navajo32...