A reader asked me how to make a gravity driven ramp-walker. I don't think I've ever seen plans. I did find some good info that I've listed below. Please add comments if you know of other good sources. We can create the best resource page on the web for making your own ramp-walkers!
I gather that this book has some info (but not a complete plan):
Earth, Water, Fire, and Air: Playful Explorations in the Four Elements
Here's a complete plan for a Tinker-toy based walker
This article shows an old wire-based walker and part of a patent for a toy with some of the key principles for ramp-walkers: http://www-personal.engin.umich.edu/~shc/CollinsWisseRuinaReprint.pdf