NASA has some great stuff on their website.

Lots of stuff about science in general since you need the basics to build up to aeronautics and rocket science. Games, images and video. Even though my son is bit younger than their target audience, we are still getting a lot out of it. They Have Nasa kids club.

Those of you with older (4+) kids will probably get more out of it than we do.The stuff on the site is free. here is the link:

http://www.nasa.gov/audience/forstudents/index.html

starfall

One of thos most educational and fun website is :

www.starfall.com

I owe a lot to this website for my son's learning about alphabets - upper case and lower case, phonics, About me (my body part, my kichen, my living room, my bathroom) etc

It has lotsa pages which makes 'reading' fun.