National Geographic's Project Noah

Project Noah - to be honest, I'm not exactly sure what this does besides the fact that it does have educational components and some golly-gee factor, per norm for Nat Geo stuff. I think, though, that especially if you're a teacher or homeschooling parent who wants his/her kids to learn more about nature, this would be worth signing up for and looking into. I've just not had the time to explore it much yet since my sister introduced me to it yesterday.

Outdoor Biology Instructional Strategies

OBIS is all about biology with experiments and info for kids of all ages.

Space Place

NASA's Space Place - science-learning fun

Art Museum Adventure

Smithsonian Art Museum's Meet Me at Midnight: learn art terms and concepts through an adventure game. Ages 7+

Cells Alive!

Learn about microbiology, cell biology, immunology, and more: Cells Alive!

Comic Math - Calvin & Hobbes

Who wouldn't love math when filtered through Calvin and Hobbes? :)

Free online textbooks

http://www.openculture.com/free_textbooks - looks like a really cool site with free textbooks for all sort of topics