Learning is enchanting! http://www.enchantedlearning.com
(Sample link) National Geographic for Kids http://www.nationalgeographic.com/kids/
Reading is fun with Starfall http://www.starfall.com
Good questions about good books http://bookadventure.com/
Puzzle fun! http://websudoku.com
www.funbrain.com/detect/index.html
Extention Activities http://www.puzzlemaker.com/WordSearchSetupForm.html
Fun online games and practice for every subject. http://www.softschools.com