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

Guitar lessons

http://youtubeguitarlessons.net/ - virtual lessons

Black Holes explained

http://hubblesite.org/explore_astronomy/black_holes/ - always a fascinating topic