Please, like you didn't expect this.
Traps are typically one shot, one dimensional speed bumps to throw at players. As such, when I do include them, I just use fire and forget stuff like pit traps, arrow traps, and brown mold sprays when doors are opened. Occasionally, I give them more moving parts. One of my favorite challenges that I've ever run involved pressure tiles on a floor that shot bolts of fire at characters if they stepped on the wrong tile (and there was no rhyme or reason as to which tiles caused fire. They rolled 50% per move.) while the enemies swung from the rafters on ropes at them. However, there is one "trap" that I tend to include in a lot of games. In fact, I've mentioned it here. The trap is as follows:
A. The room the characters enter. They come in through the door at the bottom of the room. I tend to make the door lock or be just on the other side of a chasm, or at the bottom of a slide, just so that it's hard to leave the room via the entrance.
B. A pool of murky, foul water sits in the corner of the room. There is an underwater tunnel that connects room A to a pool of water in room C.
C. Room C is full of heaps of dead bodies in various stages of decay along with an altar with an artifact that periodically animates the bodies into some form of zombie (I like Juju Zombies).
D. This is the door out. It's locked/barricaded/hidden/hard to get out of/a hole the characters have to get down and crawl through. The point is so that the characters have to work to get out of the room.
The basic concept of the trap is that the characters get into the room. They are stuck. Zombies start coming out of the water and munching at the party. There are basically infinite zombies; I tend to do something like 1d4 are animated every 1d6 rounds or something of the sort. If the zombies are really cool, they can drag characters into the water to drown them. There are four possible ways out of the room: Death, getting out of the entrance and giving up, getting out of door D, and braving the water, getting into room C and breaking the artifact to stop the zombies. Every time I've run this, it's been a blast, the characters have really been challenged, and it's been hard for the characters to get out of it. It hasn't been a trap. It's been an encounter in and of itself and not just some random hurdle to overcome.