Lightsaber from Star Wars
Knife, torch, wound cauteriser, soup warmer, skeleton key, cigarette lighter -- the lightsaber is the coolest and most versatile gadget of all time. You know you want one.
Where the hell is it?
According to the official Star Wars databank, "at the press of a button the energy contained within is liberated and forms as a shaft of pure energy about a meter long". Containing large amounts of energy in a free-standing shaft that can cut through metal would be rather tricky. So far, the closest anyone has got to anything that resembles a lightsaber are high-powered industrial and surgical lasers, which are still very cool. It's just stopping the beam so that it's a finite blade that's the problem.
I want one now!
The closest you'll get to a lightsaber today is a plastic one from Argos. It makes the noise, that's what counts. Or if you've got a you could always download the lightsaber app, which works using the phone's accelerometer and makes a satisfying noise when you swoosh the handset around.