For indoor dungeons, there is always the sliding stone block (triggered by one of them stepping on a trigger stone) which blocks their route outta there. This is more effective if the trigger stone is separated from the place where the block slides, so they hear the stone grating, but won't *immediately* realize what's happened.

One of my favorite traps wasn't exactly a trap. It was a big underground lake with a series of drains - if the controls were placed in a certain configuration, the dungeon would flood, but it would take several minutes to do so.

For outdoor traps, there's deadfalls and pitfalls. I also had a monster dubbed "fire-eater" which was the bane of my party. It was attracted by the light of the campfires and would dash over to the fire and eat it. This was naturally very noisy (especially as the players reacted somewhat predictably), and generally attracted all kinds of unpleasant attention.

One thing that a lot of DM's fail to capitalize on is that some monsters are capable of strategy and tactics. If your players are storming a fort, there will almost certainly be static defenses in place. If they are chasing a band of monsters, the monsters may well boobytrap their backtrail (read "The Most Dangerous Game" for ideas here).

IceWolf