Tutorials/Trapdoor uses

Drawbridge
If you place the hatches 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 hatch 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 hatches start.



Entrance to Underground
You can make an useful entrance to underground by using hatches. 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.

Retractable Stairs
You can make retractable stairs with hatches. Put and blocks in a stair pattern and put hatches on the side of the stairs. Use redstone wire to wire all the hatches to a switch.

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.

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

Bardoor
Hatches 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.