A somewhat modified version of tbuser's walker negative, which would allow you to make your own walking toy. I downsized the difference blocks and added a couple notes to make it more clear how to combine into your own walker.

The upper section is to be added to your object. The lower section is to be added to the 'remove' part of your object.

The walker mechanisms are available from the MakerBot store: http://store.makerbot.com/makerbot-windup-walkers.html