Soda Constructor is a physics and engineering simulator that runs in a web browser.  Using Soda Constructor you can make structures and creatures by putting springs and masses together.  The springs are then connected to a muscle wave that makes the springs expand and contract rhythmically.  With careful design of the springs you can make your structure move across the screen.

There is a Soda Constructor zoo, where users can share the creatures they have made. 

To successfully build a structure, builders must understand the concepts of centre of gravity and cross-bracing. These concepts can be discovered by trial and error.

Students can experiment with existing creatures from the zoo or built in library and explore the effects of adjusting gravity, friction and spring strength. 

In addition to the SodaZoo, there is a SodaRace competition, which consists of an online olympics competition.

Soda Constructor can be found at http://sodaplay.com/

There are a number of tutorials that show you how to get started with Soda Constructor.