From Dimitris Xanthopoulos:
Physion is a 2D Physics simulation software. It can be used to easily create a wide range of interactive physics simulations and educational experiments. Teachers may find it particularly useful since it can be used as a virtual physics laboratory through which they can demonstrate some basic physics concepts in the classroom.
Using its tools you can create various physical objects on a scene. You can for example create circles, rectangles, polygons, gears, chains, ropes, balloons and lots of other physics items. You can also connect those items using springs, pulleys, nails and other types of joints. This way you can create various scenes/scenarios which range from simple educational experiments to complex structures and mechanisms.
Apart from the classic saving/loading of scenes, Physion also supports uploading/downloading scenes to and from a central repository. Once an interesting scene has been created, the user can upload it to this repository and it will be immediately available for downloading to all other users that are using Physion.
Finally, Physion may be particularly useful to game developers since they can use it as simple level editor for their platform games.
The program has been translated into 7 languages (English, Greek, Russian, Japanese, German, Spanish and Indonesian) and is available both for Windows and Linux (32 and 64bit)