My most favorite trap from Defeating More with Less is the funnel:
Dragon 174 said:You've all heard of the confining trap; now, you'll hear of the truly confining trap. The victim blunders into a pit trap (roll his dexterity or less on 1d20, or he falls in), to discover it is shaped like a funnel, with the narrow end at the bottom. This point is so narrow that the victim will barely fit it, and will be so cramped that he will be quite unable to get out again without assistance. Now imagine a murder hole directly above the funnel-shaped pit, where an enemy can shoot arrows or bolts down on the motionless character, scoring a hit virtually every time. If missile firing seems too tedious, assume the murder hole opens into a
larger chamber, with barrel upon barrel of ghastly liquid. A few barrels full of burning oil or acid, poured down on an immobile character, will make a mockery of the advantage of high level. The fellow may have lots of hit points, but if he can't fight back, his assailant has all the time in the world to leisurely get rid of him.