Phun is a 2D physics simulation "sandbox." That description might not sound like much fun, but Phun is, um fun! It's obscenely interactive -- grab and draw and push and turn all you want. Stuff is always happening. Watch the video demonstration to see what you can do.It's currently only available for Linux and Windows (I'm running it on my Asus EeePC under Linux of course!), thought the author says a Mac OS X version is "coming soon." Binary only, unfortunately.
This will have a large (negative) impact on my productivity...
There's a game that uses the same concepts called Crayon Physics:
http://www.kloonigames.com/crayon/
The original version is free. The deluxe version is still in development.