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. Trap doors can also be used with lava underneath just in case you really don't like your foes.
Do the same with the drawbridge, then in the pit (make the pit only 1 block tall) add water, the amount of water you put is the amount of trapdoors wide. Only add water at the beginning of the sewer,if the sewer runs out of water (after 7-8 blocks) make the pit one block lower where the last water block is. Now, you can decorate it. Add mossy cobble at the sides and for added effect, connect it to a house, or make a whole water system in a town.
 Entrance to Underground
You can make a useful entrance to underground by using trapdoors. It can be controlled by Redstone using Redstone, Redstone Torches, and possibly if you want it automated, Redstone Repeaters. Obviously, you can use different objects that give charges instead of Pressure Plates.
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 Minecart Entrance
Similarly to making entrances underground you can use trapdoors to drop your minecart onto track underneath making an automatic entrance (good for keeping hostile mobs out, making it convenient getting into your base). It can be powered by the rail detector so the trapdoors will only be open for a short period of time. A minecart will not fall through a single trap door (a 2x3 trapdoor setup works fine).
 Anti Zombie Door
Trapdoors can be used as an alternative to a wooden door. The advantage of this is that zombies will not attempt to break down the trapdoor, making your house zombie free. This is particularly useful in Hard difficulty, as zombies will not go into your house.
 Retractable Stairs
You can make retractable stairs with trapdoors. Click here for tips on how to do this. Basically, you need a staircase of trapdoors, with the "hinge" on the wall, and a channel in the wall next to it for the Redstone wire. A lever can then be used at the top and/or bottom to extend or retract the staircase.
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.
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 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 built 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 close the trapdoor below you.
Trapdoors can also have aesthetic function. It's possible to create a functional and good-looking bardoor. Also, putting closed trapdoor beyond a furnace creates oven with burners.
Trapdoors used above a hopper or a water-filled cauldron can form a toilet.
You can make a distributor by making a vertical succession of trapdoors 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 object will fall from one block.
 Minecart Door
Trapdoors can be used to create a door which cannot be entered by mobs but is convenient for the player to enter on foot or in a minecart. This is done by placing a closed trapdoor on the upper block inside a 1*2 doorway and a minecart track under it leading through the door. The player is not affected by the trapdoor, and the minecart travels on the block under it, so the player can ride a minecart through the trapdoor without getting hurt and without losing speed.
You remove a block above your head and place a trapdoor there. When you hit the trapdoor, it opens and you can get in your shelter.
 Long Distance Wolf Caller
This design can be used with the distributor to make a fully functional wolf caller. By placing a trapdoor above a 1x1 pool of water with a wolf on the trapdoor, if you are the right distance away - when you push the button the wolf will fall in the water and, to prevent itself from drowning, stand up, sense you're away and teleport to you. This design is effective for people who frequently fight monsters. Add the distributor and you can successfully call multiple wolves.
 Planting Pots
If you are wanting to make a nice looking pot for your plants, then try this:
You will need a 3X3 flat space to do this properly, or at least a + shape space. Put your material (whatever you're planting the object in) in the center of the + / 3X3, Then, while walking around it, you place the trapdoors so they face each side of the brick when retracted, when retracted you will have your pot, but when not retracted, it will just sit around your plant. see below for schematics. The trapdoors can also be used as panelling for other aesthetically pleasing uses. Surrounding bookshelves with trapdoors on the sides is one other use.
 Terraforming deserts, Clearing gravel
Ever want to make that unsightly desert go away but didn't have the patience? Then this won't work because it also takes patience. However, perhaps less and it's fun to watch it go...
Dig down until you find at least 3 solid non-gravity affected blocks (i.e. not gravel or sand) below the sand or gravel you want to clear (you only need to be one lower, but going an extra level deep to start reduces the chance of you running into a sudden drop in the level of the bottom of the sand). Clear out as much as you can one level below the bottom level of sand (so there's still something solid for the sand to sit on) in such a way that you can position trapdoors under the solid blocks. Make sure the trapdoors are closed and then just start breaking the blocks above the trapdoors and watch the sand all come tumbling down at once, breaking onto objects galore!
Note: If you do this with gravel you will never get flint. But if you're planning to build a house of glass and strip mining all the sand bores you to tears, or if you're building a dungeon and just tunnelled into a desert or a glut of gravel, it's a far more entertaining way to get it all out of the way.
 Mob Spawner Trap
The AI within mobs (both passive and hostile) do not recognize open trap doors as holes, making them vulnerable for them falling off cliffs. This is a technique used in mob spawner traps to lure them into a collection zone.