PVC marshmallow gun.
The whole of the gun didn't cost $2 to make, and I'm the one who cut all the pieces and put the thing together, but it's a cheap blast of fun.
Head over to Instructables and make one for yourself. The comic above (also from Instructables) is really about how easy it is. I will make one suggested change to their instructions: a ratcheting PVC cutter makes quick work of the pipe cuts and is way cleaner and easier than a hacksaw.