www.sodaplay.com
go here and play with the most incredible artificial intelligence type thing EVER !!
you can create your own creatures from wire framed models then place them in the virtual zoo arena and watch how they sit and move !!!
It isn't the simplest system to use but if you're interested in biology and/or physics you can't fail to be amazed by the system.
I've just spent a happy making a cube with horns that wibbles along the floor (very amateurishly but most enjoyable)
It's all free and very clever stuff.
If you make one please email it to me so I can look at it !!