Alright, I have to mention this, my favorite trap that I've never had a chance to use.
The PCs enter a room with a gleaming black floor, it looks like it could possible be stone of some sort. There's some sort of condition to disarm the trap, either a panel, or a passphrase, depending on the needs of the setup. Detect magic notes some sort of evocation effect covering the floor, but poking and prodding doesn't really seem to do anything. As the party crosses the room, the wall of force that was pressing the black pudding flat vanishes, dropping everyone smack into the ravenous ooze monster. Heh, pudding.