Soda Constructor is an amazingly entertaining website... It's a constructor/simulator that allows you to build simple models with "muscles" and "springs" that you can then proceed to simulate in an environment where you control the gravity, the friction, the spring strength, and the "muscle" contractions.

They include some working models to demonstrate how the program works. Moving on from messing around with these models, here's my first moderately successful attempt at building my own moving 'creatureling.' Structurally, he's sound, and he doesn't tend to collapse under the gravity. I also managed to get the muscles to some degree work; they move properly when gravity is close to 0 and the creatureling is floating. But when he touches the ground, my poor little guy falls over on the ground after a few bouncing steps and twitches along towards the right side of the screen. I think that I need to play around a little more with different spring and muscle configurations and settings.
After a little tweaking of the muscles, I get him to not

fall over, but he's running in place, which isn't that exciting. However, this is much better than the first few things I threw together. They collapsed into a pile of twitching dots very quickly. I think his form isn't the most conducive to motion given my level of skill (or lack thereof). Perhaps if I made four legs instead of two I would have better luck.