FunBrain - The original home of Diary of a Wimpy Kid by Jeff Kinney. Full of free games and comics. Including, yes, excerpts from Diary of a Wimpy Kid. Enough said.
Lego - The official Lego site. Full of free games and other goodies.
Lego Ideas - Want to vote for which fan idea should become the newest Lego set? This site lets you do that! You can even submit your own design for other fans to vote for.
Pokémon - The official Pokémon website. Where you can find the official Pokédex and anything else you need to know in order to catch 'em all.
Sports Illustrated for Kids - Online version of the Sports Illustrated for Kids magazine.