The X-Net is a simple but very effective device which will stop a five ton truck in under 40 feet. It comprises a net with spikes at the leading edge. Lay it out on a road (or dirt track, or your neighbor's driveway) and wait for the looks of surprise. The barbs puncture and then grab the tires and the net wraps around the wheels, stopping the vehicle in around the same distance as a regular emergency stop.
At the moment Qinetiq, the manufacturer, are supplying a hand deployed version, and although it only takes 20 seconds to set up, there will soon be launcher which will get the job done in three seconds.
Best of all, there is a video of the X-Net in action. We couldn't embed it here, so you'll have to click through to the site.
Product page [Qinetiq via Coolest Gadgets]
Video link [Qinetiq]
