My favorite trap for a relatively low-level party:
The PCs come through a door to enter a huge room with a vaulted ceiling and a checked floor of red and white stone. In the center a pedestal rises with something on it the PCs want/need (depending on adventure). When they grab it, a skeleton raises on each red tile. The twist: red tiles cast inflict light wounds on anything that steps on them (let the players figure this one out themselves! If it takes awhile, give them hints). What ends up happening is the PCs get injured when they move and the skeletons get continuously healed, making for a rather tough battle with usually-easy skeleton mobs.
I have used that tactic in 3.x times, but now in 4e, I haven't found such thing as inflict wounds.