Atlapedia Online http://www.atlapedia.com/
Fact Monster: Homework Center http://www.factmonster.com/homework/
Homework Spot http://www.homeworkspot.com/
Infoplease: Homework Center http://www.infoplease.com/homework/
IPL TeenSpace http://ipl.si.umich.edu/div/teen/
Math.com http://www.math.com/