I'm pretty sure they won't let you use an electric bike in the Tour de France, so why should we tolerate paper airplanes with tiny electric motors? Because it sounds pretty awesome, that's why.
The Electric Paper Airplane Conversion Kit includes a tiny propeller powered by an even tinier electric motor, that you can add to any standard paper airplane of your own design. The propeller sits at the back and pushes the plane, keeping the craft from becoming nose heavy.
The real stroke of genius is that instead of a heavy battery, the motor is powered by a capacitor that holds just enough juice for 90 seconds of flight. A separate battery pack charges up the cap in about 20 seconds, so you'll be ready for another flight.
The Electric Paper Airplane Conversion Kit is available from ThinkGeek for about $20.