I've often thought a simple time delay fuel pump switch would be a good marketable device...
It basically lets the car get stolen, but runs out of gas half way down the block. They split and you get your car back.
Nice for car jackers too...You get out, they get in... and you watch them run out of gas as you call the cops.
Never saw one on the market, but wouldn't take much to throw one together.. could plug into pump fuse receptical for circuitry.