We're using the D&D 4e rules, and the players get to pick a PC at random, and then go into our dungeon facing overwhelming odds. PCs are killed left to right, and the players love it!
UPDATE: The Dungeon of Doom is a simple and very popular concept. I had the idea several years ago and together a group of friends put together a killer dungeon for D&D 3e. We had maps, kill markers, overwhelming monsters, adversarial dungeon mastering, and low level PCs. And dropin. Anyone could play, and it took about 15 minutes for their PC to get whacked. And people lined up in droves and droves.
The freewheeling manner of play, one of the basic philosophies behind my own gaming style, is a real draw. And it seems to work equally well for D&D 4e. We're running into some things that slow down play, such as Analysis Paralysis, but a firm DM hand brings things up to speed when things get bogged down.
We have one DM, one rules assistant and one player coach. One huge map, minis for the PCs and minis for the monsters. Dice, rulebooks and stuff like that. We've laminated the character sheets, so players can write on them with dry erase markers.
Works like a charm!