Traps are best when the players know they're there. Otherwise it's just a gotchya, and that has very little fun value. Traps should cause a choice, even force one, even if the choice is to just avoid it. They should also inspire creativity.
One of my favorite traps involves a door. To open it, you have to turn a wheel or pull a lever thats well away from the door. There is a matching lever on the opposite side of the door. The door will only stay open as long as someone holds the lever. This works really well in a long hallway.
This, in itself, isn't much of a trap, until you make the players hurry. Have something big and nasty chasing them, and suddenly it's a big deal who's going to hold the door. Does the one holding the door open stop to defend himself? Because, of course, it takes both hands to hold down the lever, and by letting go, the door slams shut, trapping him in... but if someone holds the door open, the monsters can come through...
Kind of a DFD sort of situation. Try to save your friend, and maybe doom everyone?