Energy Kids - Contains a wealth of information and ideas about various kinds of energy, games and activities, history, saving and using energy, and much more!
Exploratorium - Science activities you can do from home, put together by the Exploratorium, a hands on science museum for all ages.
National Center for Educational Statistics Kids' Zone - The NCES Kids' Zone provides information to help you learn about schools; decide on a college; find a public library; engage in several games, quizzes and skill building about math, probability, graphing, and mathematicians; and to learn many interesting facts about education. Both fun and informational!
Steve Spangler Science - All kinds of great science activities that you can do from home.
Young Ocean Explorers - A website full of great videos and information about the ocean, made by a father and daughter team from New Zealand.