Mathematics
http://www.primarygames.com/math.php
http://www.mathblaster.com/free-game.aspx?pid=googpd&cid=Content
Science
http://www.primarygames.com/science.php
http://pbskids.org/games/science.html
Reading
http://www.abcya.com
http://www.primarygames.com/reading.php
Social Studies
http://www.primarygames.com/
social_studies.php
http://www.netrover.com/~kingskid/
social.htm