Mekanimo is a 2D physics simulator. In minutes, you can create simulations, interactive games and puzzles, dazzling graphics and share them with others through the Internet.Mekanimo really surprised me with its professional-looking user interface. It allows for the creation of worlds with customizable scripting, various plots, and so many other things. I enjoyed trying out the exhibits like the marble pump, which require you to copy and paste the code into Mekanimo for now -- I think the author Fahri has been putting much more into the application itself than its website.
It's still got a ways to go and unfortunately is (for now?) closed source, but keep an eye on it. I expect it to turn into a really nice and polished learning/design tool some day.