Here are some links that I have found to be helpful in my classroom
http://kids.nationalgeographic.com/kids/
http://www.highlightskids.com/
http://www.newton.k12.ks.us/tech/fourth_grade_internet_activities1.htm
http://www.exploratorium.edu/mind/judgment/cuteify/v1/
http://www.museumca.org/goldrush/
http://legacy.mos.org/sln/Leonardo/LeoHomePage.html
http://www.mathapprentice.com/