I was always impressed by the ability of Star Trek characters to set their phasers to ‘stun’ and thus provide bellicose aliens with a proportionate response, rather than uniform, maximal force.
Today’s invention is a related adaptation to existing handguns (rather than to specialised riot control devices).
This would take the form of a rotary valve which would allow the weapon user to preselect the area of a set of circumferentially-arrayed vent holes through which some of the propellant gas from a cartridge could be discharged.
No venting would propel the bullet at normal speed; the fully open setting would allow almost all the gas to escape and thus reduce the muzzle velocity of the projectile to a very small, and certainly non-fatal, level.
Here is a related development that I didn’t come up with. Why didn’t I think of that? http://www.gizmodo.co.uk/2011/12/the-compbullet-reduces-recoil-with-rocket-vents/