Although the behemoth known as a spud cannon is fun (see below), I also like the smaller, and safer, eponymous device which is basically a squirt gun except that you load it by shoving the tip into a potato, and you fire it by pulling a handle which compresses the air within the chamber enough to expel the little bit of edible starchy tuber. Not nearly as satisfying, but it's great for doing live-action Quake, namely because it won't actually kill anyone. It is to a blaster as a spud cannon is to a railgun.
Real-life Quake could be fun...
Spud Cannon
Also known as spud guns, these are frighteningly easy and cheap to make. My brother and I made one by taking a 3"x2" 'why' PVC junction (used to attach a 2" PVC pipe to a 3" main), putting about 3" of 2" PVC in the branch, putting a cap on the end of that, and affixing a Coleman lantern flint into the length of pipe. This became the handle and trigger. For the back of the 'why' (which is the mixing chamber), we attached a cleanout adaptor (used to attach a screw-on cap for the purpose of, well, cleaning out the pipeline), as well as a screw-on cap. On the front, we put a 3"x2" flange bevel (allows you to put a 2" pipe into a 3" fixture), and about 2.5 feet of 2" pipe into the bevel. Before attaching the pipe, we cut a pair of notches in the end and put a cut-off nail across it (this serves as a stop to keep the ammunition from falling into the mixing chamber). After that, we filed the tip of the pipe, making it sharp, so that it could cut a potato. Everything gets sealed tight (except the end cap, which needs to come on and off). We also took an extra piece of 2" pipe, cut a square hole in it, and attached it to the wrench-hold on the cap so that we could put it on and take it off more easily.
To load: shove a potato onto the end of the muzzle, turn it to cut it, and push it downwards until it's a round chunk in the barrel. Use a long stick (such as a broom handle) to gently push it down until it hits the nail. Turn it over, open up the back, spray in cheap hairspray (I recommend Aquanet, as it's cheap and very high in propane) for a few seconds, immediately cap.
To fire: aim, then with a flick of the wrist, spark the lantern lighter. You should feel a satisfying recoil, hear a resounding pop, and see a chunk of potato be propelled at about 120 MPH into low-earth orbit.
Great for ringing in the new year/millennium.