I've always thought of traps as a sort of reading comprehension challenge. Before the PCs can bring their mechanical resources to bear, the players have to first realize that there's a trap in the area. I think that's why the most successful traps I've implemented follow this sequence:

1. GM describes the area, providing a bit of trap foreshadowing.
2. The players listen attentively.
3. Either:
3a. PCs miss the clues in the room description and set off the trap.
3b. PCs realize something is amiss and respond dynamically to the situation (as opposed to rolling Spot/Perception/etc. checks at apparently empty squares).

For me, the mini-game of trap finding is all about players trying to outguess and outthink the GM. Case in point.

Does that sound like a familiar paradigm to you guys? What are the most successful (and by "successful" I mean "players had fun interacting with them") traps you've used?