My most notorious trap in the campaign thus far was the harpoon trap in the Temple of Karophat, the God of Misuses of Technology (including building Traps).
The trap was pretty simple in concept, but quickly became complex in its actual effects.
There was an array of pressure plates in the temple's inner sanctum, which anyone crossing had a chance to trigger. The Champion (think Paladin) went in first, and luckily managed not to step on a pressure plate. So the Explorer walked into the room next, and immediately stepped on two in the space of a stride. This caused the Explorer to be almost simultaneously struck by two harpoons fired from recesses hidden in the ceiling. Both hit and embedded themselves in his flesh. The harpoon's lines were attached to reels that then pulled the victim up to the ceiling 20 feet above. The two harpoon's engaged in a tug of war, one lost and was ripped from the flesh of the Explorer doing rending damage. The remainder hurled the Explorer against the spike covered ceiling with great force - essentially the Explorer fell upwards into a spiked pit trap - pinning the Explorer to the ceiling and impaled on a number of metal spikes. Amazingly, the Explorer managed to live through this, but was not in that good of shape.
At about this time, a thick vapor begin to appear in the room, blocking vision. It was in fact only an Obscuring Mist cast by the invisible low level priestess who was in the room, but the party assumed the room was filling up with a poisonous gas.
The rest of the party panicked. Nobody wanted to risk moving about the room after the above description, and no one was sure how to get the Explorer down. However, the Explorer was a bit of a skill monkey and managed to extricate himself from the Harpoon with a lucky Heal Check, grab onto a chain that was suspended from the room, and leap down without too much harm. He then bolted for the door...
And promptly triggered a third harpoon, which also hit, and also hurled him back into the spiked ceiling.
By this point, the Explorer was now in negative hit point territory, but the rogue recovered his courage climbed up to the ceiling, made his away across the network of chains, helped the cleric get up by attaching a rope, and between the two of them, a large number of skill checks and some judicious spell use and healing, managed with the Champion's help to get the Explorer to the ground alive.