A switch that breaks a wire from the
ignition of a car, rendering the car inoperable unless the switch is
tripped before starting the engine. A good system of
theft prevention, except that your car may still be broken in to, only for the intruder to find that it can't be started. To circumvent the switch, you'd need to find the origins of the ends of the wire and
re-wire it yourself, which hardly anyone would do if they were sitting in someones' car about to steal it. However, it's only safe as long as it's hidden. If the thief finds the switch, the car is just as easy to break into as without it.
This leads to a whole lot of
wacky situations, such as the guy who had one in his
boot - everytime he started his car he had to open his
trunk and
flick the switch. Someone else who was a bit
paranoid had three switches hidden in various places in his car, it took him about two minutes to flip them all.
You can buy the switches from nearly any
automotive or
electronics store, and wire it in yourself, even if you don't know how to
solder. The switches are only a couple of bucks, and are well worth it if you're bothered about security at all and have no alarm or
engine immobiliser.