Phun is a physics sandbox toy and educational tool. Using its simple tools and powerful physics engine, you can build complex machines or just have fun! This video should give you an idea of what you can create in Phun:
Phun is not open source, but can be downloaded for free. You will need to have hardware accelerated graphics to play it.
Visit the download page and download the Linux version. Extract the archive anywhere you like, and double click the file phun to run it. No installation is necessary.