Well, the first pneumatic prop I ever built was with a screen door cylinder, with plans from hauntedillinois.com. http://www.hauntedillinois.com/pneumatic.php
It's...not really the safest thing in the world, I've heard that a couple of these screen door cylinders have malfunctioned, I don't remember the exact story, just take precaution when using compressed air.
From here I used the plans on phantasmechanics.com for the grave jumper. http://www.phantasmechanics.com/jumper.html.
OH and I just remembered, a prop I made for a friend of mine, probably exists somewhere on the web but it was a custom creation for me. I can't really draw it out at the moment, but in theory it's a popup powered with a bungi-cord. A pole, such as a broomstick, is secured on a hinge to the center of a long board. A bungi cord is attached to the center of the pole and one end of the board....
....actually I'm struggling here trying to explain it, I'll have to draw it up. If you're interested let me know, I'll open up paint and draw something up. In summary though, the pole is locked down, and when released springs up.