Dave at Pop Sci has a fun motorized paper airplane you can build, he writes –
The single-sheet paper airplanes you made when you were a kid (or perhaps last week in your cubicle) were, well, kid stuff compared to this beefed-up paper craft, which uses an electric motor from an old pager or toy powered by a big electrolytic capacitor. Known as a “Gold Cap” and made by Panasonic, the capacitor charges from a battery pack and then metes out a steady flow of power to keep your propeller whirring for about 10 seconds–just long enough to send the flyer airborne and carry it between 30 and 100 feet. (You can also add a gearing system to squeeze out even more flight time.) We’ve created plans for an eAT-6 “Texan” airplane and put them here and here to get you started. Just think twice before blindly launching this one soaring over the wall of your cube.