Inspired by a recent Penny Arcade strip here, I thought I'd post my favorite D&D trap.
It's from "A3: Assault on the Aerie of the Slave Lords."
Cunning Gelatinous Cubes
"As soon as the first rank of the party walks onto the 10 foot x 20 foot area (marked by the dotted lines) a 1-foot-thick slab of stone tilts, spilling them into a pit on top of a gelatinous cube. Characters in the second rank must save vs. Paralyzation to avoid falling into the pit. [Frost - Note: only the *second* rank gets to make a save.] Every round, a character who is in contact with the cube will take 2-8 points of damage and must save vs. Paralyzation or become paralyzed for 5-20 rounds. In addition, once per round the cube will extend a pseudopod and strike an opponent up to 15 feet away. The pit is 10 feet wide, 20 feet long, and 5 feet deep. The cube has altered its shape to fits this area."
To make matters worse, there is a second cube in the room that sneaks up on the party while they deal with the first. Glorious.
When I DMed that module back in high school, I do recall it messed up the party's monk pretty badly, but I don't think anyone died. I think it's the evil simplicity of it that appeals to me. Well, that and it involves gelatinous cubes (and cunning ones at that).