http://www.instructables.com/id/Easy-to-build%2c-easy-to-use%2c-water-bottle-launcher!/

This one looks cool and safe.





http://www.instructables.com/id/PVC-Air-Cannon/

Basically 2 pieces of PVC pipe with a ball valve in the middle.
One end is open, one end is closed and has an air valve in it.


I'm going to make one like this...out of maybe 1" pipe...not HUGE 3" (potato size)
Mine will shoot these sort of 'projectiles' (about the size of a piece of big chalk).


Air tank valve/tire valve. $1.50 at fleet farm.
Screws into the PVC (after drilling a proper hole...of course).

Ball Valves - 3 to 5 bucks depending on size.

Some PVC pipe and fittings and a cap are pretty much all you need...plus the PVC cleaner and glue.


http://www.spudtech.com/ has ideas.