Sorry for the awful title to this post – but I have been inspired by “John Travoltage”, the most fun Physics interactive I have ever encountered. I should warn you – you may lose hours of your life to this joyful game.
On a more serious note, this website has a whole bunch of “Fun, interactive, research-based simulations of physical phenomena from the PhET project at the University of Colorado”.
And the best thing? They’re all FREE.