The Autoloc Flame Thrower clips onto your tail pipe and sparks fuel that should have been ignited and consumed in your engine, sending flames up to 20 feet behind your car. (In my experience, you'll be lucky to get more than a couple of feet of flame, but it's still impressive.)
They're "designed for carbureted vehicles, but can be used on fuel injected vehicles with additional modifications and parts." What they mean is most modern cars aren't coughing out enough gasoline through the tail pipe to fuel a flame so you'll have to do something silly like make your engine less efficient or run in a second line of fuel to get these to work.
A single exhaust system is $125; dual exhaust is $200.