I don't know how many people still play Doom 2, but here's the best Doom deathmatch map ever made. (If you have trouble with the link, try clicking on it and saying “Save link as” — the “WAD” file format seems to confuse my browser.)
What makes it so great? Well, for one thing this has got to be one of the most thoroughly playtested levels in the history of video games. This map is basically what my friends and I did instead of high school. We played this map constantly as I was building it, and over months and months the level evolved to foster ever more elaborate wheels-within-wheels of strategy. Over a decade later friends of mine who've already spent hundreds of hours playing this map are still playing it.
So it's a prison, right, and there are all sorts of buttons you can push that will open and close certain doors and seal off certain areas and activate certain traps. The basic layout is a ring, and if you're in a stronger position you can create a block in the ring and then circle around the other way. Alternatively, if you're in the weaker position you can shut a door and buy yourself some time as your opponent has to circle the ring. If you find yourself trapped in one of the cells, you can either hide and lie in wait for your opponent, or if you've got a rocket launcher you may decide a strategic suicide is your best option. The level discourages camping, as if you do your opponent will begin locking you away from all the best weapons. And to really excel you've got to learn all the ways into and out of the secret passage/torture chamber.
To play it you'll need a copy of Doom 2 installed. Then copy the kprisn36.wad file into your Doom directory and type: doom2 -file kprisn36.wad
And make sure you're using "old deathmatch" rules, otherwise all the strategy just goes right out the window.
ETA: Keith Burgun, lead designer of the awesome 100 Rogues iPhone game, writes: "I can totally vouch for this as one of the greatest Doom deathmatch levels ever devised! I absolutely love KPrison, Dave, so much that I ripped several elements of it off in various maps of my own. I even had one map that I called 'KSFALLS,' and I think I just added the K because I wanted it to sound like it was like kprison (The 'S' was because it was 'slime-falls')."