Adults: As always, please evaluate any media yourself if you are worried about content.

General Information
Fact Monster
National Geographic Kids
DK Find Out!
Encyclopedia.com
Merriam-Webster Dictionary
Biography.com

Homework Help
Brainfuse - Free online tutoring with live tutors, practice tests, and more.
Khan Academy - Free online lessons for all ages, sorted by grade and subject.

Games
ABCya!
PBS Kids - Games     
Sesame Street - Games
Cool Math Games
Mr Nussbaum
Disney Junior - Games
National Geographic Kids - Games

Things to Do
Highlights Kids
Smithsonian Institute for Kids  
Scrap Coloring
Science Bob