The real deal uses gyros to maintain attitude and compound pantographs for articulation, and might be a real pain to build from scratch, but for someone who loves a challenge...
I saw a professional wedding shooter use a neat trick once that I subsequently tried, and it works pretty well as a "poor man's steadicam".
He had a very nice pro-sumer (3 chip?) DVCam on a medium size tripod, with maybe 4 foot extended legs. When he was ready to go "handheld", he just picked up the tripod and camera using the center adjustable vertical center shaft, and continued shooting using the LCD viewfinder. The weight/mass of the tripod gave him a lot of general motion stability, and all the weight of the legs below the center of gravity acted like a big pendulum, keeping the whole rig essentially vertical. He could walk around in the crowd, do up/down and sweeping pan moves that were very steady.
The key is the weight of the tripod was probably several times that of the camera alone, so this works best with the newer lightweight cameras. It's a nice simple all-purpose shooting aid, I think.