side view | T[ _| D [ | = iron door D = dispenser _ = stone pplate T = torch [ = wall not shown: wiring between plate and dispenser; underground
It looks like there's stuff behind the door (making a 1-deep hallway to the side at the torch enhances this effect greatly), and the pressure plate opens it. You can't see the dispenser unless you stand on the Pressure Plate. Because it's stone, you can't drop an item on the plate to test it. Player walks on the plate, door opens, dispenser shoots an arrow in their crotch. Also, if they walk in, they are trapped (door opens only one way). Best thing to do is make the hallway of Furnaces, floor of Smoothstone. Furnaces take ages to break. Make a 15-deep drop under the smoothstone between the door and dispenser so they fall to their death, for good measure (1 arrow will ~never kill).
If you don't want to walk into traps like these on SMP, carry levers at all times and use those to open doors. Plates on the floor can be wired to countless of traps!
I've also made a more elaborate, and deadly, trap. Fairly easy to verbally explain I think.
Lava walls on two sides of a 5 step long, 1 step wide walkway, arrow dispensers on one side behind the lava walls. First step is clear. Second is a pressure plate. Third and fourth have an arrow dispenser aimed at them. Fifth is clear.
They trigger the plates, get hit by an arrow, and are shoved into the lava falls. PANIC!
Bonus points: shoot arrows from the back as well. Delay side traps with a double inverter. If they just camp on the plate, the arrow from the back will shove them into the first side arrow. Arrows from the sides fire with a small delay, and thus hit later if at the same distance. First arrow puts them in line of fire of side-arrows. Also more damage.
top view + bonus points L L L L D L L D L_L L L LLL D _ = stone pplate L = lava D = dispenser not shown; redstone wiring to dispensers. double inverter to 2x D on side.
Minecraft is the best.