Algodoo

A switch blade with a 'Buck knife model 119 special' kind of blade.
If fixated to the background, it works very well. If unfixated, it doesn't work as well. It's too fragile I guess.

For this knife to work, the sim.frequency needs to be 500. If it's not 500, open the console by pressing F10 or tilde (~) and type in sim.frequency=500.

Controls:

Auto:
Enter/Retu-
rn - Open the knife.
Back Slash Key - Release the blade lock.

Manual:
Pull down on the top, rubber switch - Open the knife.
Press in on the silver button - Release the blade lock.

This knife uses no scripting, and no hinge motors, except for automatic opening and assisted closing (unlocking the blade lock so you can drag the blade closed).

If you don't believe me, set all the hinge motor settings to default, and look at the oncollide and onhitbylaser for all of the geometries.