This is the design for one of four legs on my second four legged robot. The first was a wooden monstrosity that's not really worth going into detail on.
The second was much better and actually walked under manual control. Each leg can go up and down linearly and foreward/back on a hip bearing. Unfortunately, after adding computer and other electronics it became too heavy to move itself and although it was thinking the right things and driving the motors accordingly, it need a little tap in the right direction.
My report written for the Science Fair on the 4 leg robot is available in
PostScript (50k) and Text (12k).