I built a simple one with a small device that can hold those 16 gram CO2 cartridges:
I disassembled the unit and replaced the fittings so it'll work with a standard keg. It probably cost me $40 for all the fittings, pipe and the unit plus a few cartridges, and was super-simple to build up.
It is unregulated so you have to give it a quick (<1 second) squirt every 5-10 beers. I get through a keg with about 5 cartridges. It works really well for tailgating, bbqs, etc. But it isn't idiot-proof... don't let the peons use it or the beer will start flowing at about 50PSI and spurt everywhere