Amazing Space 
This site is simply, well, amazing. There are tons of cool things to do while exploring their Online Explorations:
Planet Impact — Discover the force of gravity and pelt Jupiter with a comet.
Mission Mastermind — NASA needs your help planning Hubble Servicing Mission.
Galaxy Hunter — A cosmic photo shoot.
Comets — Whip up a batch of comets.
Comets, Myths and Facts — Explore comet facts and myths.
The Truth About Black Holes — Learn all about black holes.
And that is just a handful of the things you can do on this site. You can also get homework help, and Capture the Cosmos.
My favorite things were: Capture the Cosmos, a totally cool online adventure where you can learn all about the things that make up the cosmos. The Black Hole section was great. I learned so much about what they are and how they form.
This site is Out of This World!