Tutorials/Trapdoor uses

Drawbridge
If you place the trapdoors in a trench in this fashion:

You can create a fairly good drawbridge. You can extend it lengthways, but not widthways. However, it's possible to walk on the ledge of an opened trapdoor if there is enough space above it. It's possible, however, to create a drawbridge in this fashion that is not vulnerable to ledge walking, so long as there is an overhang 2 blocks above the ground before the trapdoors start.



Entrance to Underground
You can make an useful entrance to underground by using trapdoors. It can be controlled by Redstone using Redstone Dust, Redstone Torches, and possibly if you want it automated, Redstone Repeaters. Obviously, you can use different objects that give charges instead of Pressure Plates.

Full Hidden Wire Tutorial Part 1/2

Full Hidden Wire Tutorial Part 2/2

Making of trapdoors dungeon entrance

Retractable Stairs
You can make retractable stairs with trapdoors. Make an empty space with the shape of a staircase. Place the first trapdoors on each side to serve as the first step, climb on top of them and put the next steps by placing trapdoors 1 block above the ones your standing on. Once your staircase is done, wire them all together by creating a redstone wire path carved in the wall on each side.

Traps
Traps can be built by placing lava or water underneath trapdoors and wiring redstone circuits and pressure plates to them. This can be used as a 'reverse drawbridge' or underground to trap mobs or other players.

For example: Minecraft Beta 1.6.6: Trap Door Hidden Wiring Trap



Launching Pad
It is also possible to create a minecart launching pad by placing two trapdoors side by side. Now connect the trapdoors with a button/lever and one redstone wire. Then build 2 ramps with minecart tracks. One should lead to your launch pad and the second one should be build under your trapdoors. If you build 2 minecart launching pads side by side, it would allow you to create a minecart racetrack. Just put your minecart on the top of the upper ramp, enter it and wait until you're on the trapdoor and press the button to start.

Trapdoor Elevator
You can make elevators with trapdoors. This is done by placing trapdoors one block apart from each other, straight down. You open the trapdoor below you to allow for extremely speedy descent. To ascend, jump and open the trapdoor above you. 

Bardoor
Trapdoors can also have aesthetic function. It's possible to create a functional and good-looking bardoor.



Distributor
You can make a distributor by making a vertical succession of trapdoor and link them with redstone. Use a button to power the circuit. Just place an object on each trapdoor and when you'll push the button, each objct will fall from one block.