Websites
Math
http://www.eduplace.com/math/brain/ - math brainteasers
Reading
http://www.scholastic.com/kids/
Social Studies
http://bensguide.gpo.gov/index.html - learn about the U.S. government
http://www.funbrain.com/where/index.html - practice your states and capitals with this game where you can choose your ability level
Science
http://www.learner.org/interactives/parkphysics/index.html - amusement park physics
http://www.billnye.com/ - Bill Nye the Science Guy!
http://www.mos.org/sln/Leonardo/LeoHomePage.html - Leonardo da Vinci
General
http://www.sheppardsoftware.com/index.html - activities in all subject areas
http://www.funbrain.com/index.html - choose your game to fit the subject