My two favorites are the pit-you-could-over-but-its-blocked (i.e. a pit with a wire or wall of force across the corridor) and the triple spring trap I will explain below. The pit trap had the advantage that it would not show up with the old version of the clerical find traps spell (most 2nd ed. parties I ran always had this running). I'm sure one could argue the point, but a wire strung across the top of an open pit doesn't really do anything. It just leads you to believe you could jump over the pit if you don't see it. The triple-spring trap is one I have used on several different parties, and it has always wacked them a bunch of times before they figured it out. The way it works is: a pressure plate on the floor (or other trigger) triggers large blade (or other damaging thing) to swing out from the wall about waist high, hitting the character for some damage and knocking him back. Triggering the trap again causes a similar blade to swing out about knee high. A third time triggers a blade to swing out about 4-5' off the ground. Every time I have employed this, the party gets hit, tries to crawl under and gets hit again, and tries to jump over and gets hit a third time. Some figure it out by then, some need 1-4 more whacks before they get it. Not the most lethal or impassable, but it realy irritates the party that some dwarf hundreds of years ago know exactly what they were going to do.