Here is a flash animation of Theo Jansen mechanism which is a very interesting linkage systems. This ingenious mechanism is very good example of four bar mechanism described before. In this mechanism 4 four bar arranged such that a rotation movement is converted to walking pattern of a four legged animal. You can see videos of this mechanism in action by doing google search and entering key word "Theo Jansen"
There are only limited arrangement of bars which gives a good imitation of a four legged animal walking pattern. By changing sliders you can change lengths of corresponding bars. Also you may move center green ball or right red ball to have different arrangements.
Please note that this is very unstable animation and some arrangement destroy animation and there is no going back, if animation act strange, just re run it to turn to initial arrangement.
I enjoy watching videos of Theo Jansen mechanism and playing with this animation and I hope you will enjoy too.