In the classroom I limit the websites that my elementary students can explore. Here are a few.
Kids' Games, Animals, Photos, Stories, and More -- National Geographic Kids
An excellent typing game.
BBC - Schools - Dance Mat Typing - Home
Kids Games, Kids Movies, Kids Music, and More - Yahoo! Kids
Discovery Kids :: Home Page :: Discovery Kids
Various subject games. Students enjoy the Math games.
FunBrain.com - The Internet's #1 Education Site for K-8 Kids and Teachers
The kids love this one, plus it makes a difference.
FreeRice