http://www.nowykurier.com/toys/gravity/gravity.html (http://www.nowykurier.com/toys/gravity/gravity.html)
slightly biased cos i'm a physicist but this is really really addictive! make orbits, binary star systems and all sorts.
i've wasted a good hour or so on this...
(http://i47.tinypic.com/30tkivc.jpg)