Time to give an indie programmer a bit of press. Yusuf Tor has created a fun little game called Space Orbiter, which can help you learn and observe the effects of gravity and velocity (and the spacey music loop is not bad either). Though it's not a high-end 3-D game (only 19MB in size), I think it would be mucho fun for the kiddies, and hey...it's only 99 cents! The concept is pretty simple. You position your orbiter, set a speed vector, and hope that your settings result in the longest trip round and through the planets.
