Inspired by corben33, I had my students design CO2 cars using Autodesk Inventor. We then printed them on a Makerbot Replicator 2 and entered them in our county fair. We had a lot of fun and learned a lot. Next year we will be even faster! I uploaded a video of their races at http://youtu.be/2IstuGfeM58

The hole for the CO2 cartridge can be printed without support. I had to print the ones with a spoiler with support and then clear out the cartridge hole. The small one, (the fastest) had a nose cone glued to the front. Printed at 5% fill, held up with no problems.