sodaplay (also known as sodaconstructor) is a physics simulation. Its generally accepted purpose is to create moving models, but it can be used to construct many other things, notably roller coasters and art.
Sodaplay models consist of masses which are connected by springs (bars). These springs can be made into muscles, which expand and contract rhythmically. Just a few properly synchronized muscles can create a complex and lifelike motion. You can also construct fixed masses, which are completely immobile. These can be used as guidelines, to make sure that certain pieces of your model are the same length.
Many different types of models are possible, limited only by their creator's imagination. Indeed, one of sodaplay's great strengths is its replayability (some might call it potential for addiction) . Once you've finished a beautiful masterpiece, the game is not over; now you are free to apply what you have learned and build an even better model.
Sodaconstructor lives here: http://sodaplay.com/constructor/player.htm
Support forums are here: http://sodarace.net/forum
Here is a tutorial site for beginners: http://sodaplaycentral.com