They provide additional homework etc...
Track down resources by topic and age.
http://mckennasmousepad.com
http://www.nationalgeographic.com/ngkids/
http://www.harcourtschool.com/activity/thats_a_fact/english_4_6.html
www.primarygames.com
http://www.aplusmath.com/
http://www.eduplace.com/
http://www.gamequarium.com/