Did you have Phun lately? No? Not sure? In that case, check it out! It’s great!
Phun is a “2D physics sandbox”. You can draw objects with your mouse and then have nature have its way with them. Even if that sounds stupid or a waste of time to you, check the flash video on the home page out. If you played with Lego or Fischertechnik as a child, this one is definitely for you (and since that was probably already a few years in the past: for your children, too).
On the site, you’ll find links to insane machines people have already built and of course, there is already a huge amount of videos on YouTube.
And last but not least, there is a Windows and Linux version (32 and 64bit!).