http://gravit.slowchop.com/

Simulate Gravity doesn’t mean being able to walk to the shops like you’re Neil Armstrong on the moon.  Its a clever application to model the gravitational effects of sub-atomic particles, so useful for Physics students perhaps but another of its selling points is that it "looks pretty"

It’s fascinating to use and could easily interest kids who want to take science further.

. . .now, where are my moon boots?