Type: unknown, unsure whether Feature or Bug Exploit
Ever wanted to put an hammerite or a zombie down without officialy dealing damages? Try this:
Grab a hammer (one of those that you can produce in Cragscleft). It will not be added in your inventory, but be displayed on the center of the screen (as if you were carrying just another piece of debris, or a crate or other climbing utensil). Hide yourself, and wait that a target passes by.
You can either run now towards him, and throw your hammer when you are at full speed (this will increase the damage the hammer deals), or just throw the hammer at the target without moving. This technique is one of the ways to kill someone in Expert difficulty, without getting blamed of murder. In fact, it does not even count as dealing damages at all - the ideal way to get rid of annoying enemies in disliked missions.
You need for a Hammerite about 2-4 hits with the hammer to kill it, many hits for a full-alerted Inquisitor, and 4-5 hits for an unalerted Haunt. If you throw the hammer while sitting on a beam i.e. in RTC, you will be able to kill a Haunt with one single hit.

Type: Bug Exploit
This movement technique is rather a bug exploit than a feature, so don't rely on it.
Apparently, if you start running, jump, and jump again the very moment you hit the ground, your speed is increasing above the official limit. By "kangaroo-hopping" this way, you can move very fast in one direction.

However, this is both noisy and very un-thievish. It may be fun if you're just want to do weird things and need to gain some speed before whatever.
This works best if you have long corridors or streets before you; as soon as you turn or miss a jump, you slow down.
Take care not to run into something - you easily get so fast you can kill yourself in the crash.

Type: Bug Exploit
Now that's a bug exploit!
With NPC jumping, you can propel yourself though the air like, err, with a rocket jump in a certain game series. Without fireworks. No fire arrows or explosive kegs involved.
It works like this:
Get close to an NPC (non-player-character), preferably one who hasn't noticed you or won't fight you. Get very close, lean just over his (or her, or its) "shoulder" - and jump. SDM described this as "sort of bend around the enemy, and hit jump while still leaning". Don't move when leaning.
It may take you some training to get this right, so you may want to try it on the keepers in Training, which won't fight you or run away.
Warning: after going up, you're inevitably going down. And suffering damage accordingly. A good NPC jump can show you a whole town, and kill you afterwards.
To get into special places, you can aim a bit by facing in the appropiate direction. This is a bit complicated, so it's best you try it and get a feel for NPC-jumping yourself.

If you don't know how to lean forward, you should definitely go to the configuration screen and assign this function. It can be very useful.
It has been reported that you can get occasionally get a similar effect with rope arrows. Since that is harder to reproduce, it's also less fun.
The intensity of this effect depends on the speed of your computer. Don't complain about that, it's a bug, not a feature, so it's supposed not to be there at all. And one more thing: you can't "rocket-jump" with fire arrows. If you don't believe that, try it yourself...

In Training, it is most prolly the easiest place to train the NPC-jumping technique. Set yourself the goal to jump to the windows to the left and the right in the room where you learn to move unseen, make sure you've about 1-2 hours spare time, and then begin.
I found it easiest in the following way:

• Stand directly infront of the Keeper and face him,
• turn 85° right (turn left wroks also, but somehow I prefered the turn right-technique. Maybe because I am a leftie? dunno)
• Lean forward, that means, lean your head not into him (then you will only climb on his shoulders when jumping), but a tiny bit right of him.
• Look at the floor, so that you will not only fly through the room (but flying through the room, hitting the wall and recieve some damages might also be fun )
• Turn slightly left to face the Keeper, and jump in the same moment.

From then on, you've absolutely no control about where you are flying to, at least I was unable to do anything else than watching my flighty in stunned amazement.
One of the most silly aspects of that NPC-jumping is that the NPC is pushed around with your attempts to use him for your jumping.