Math:
www.kahnacademy.org
www.ixl.com
http://nlvm.usu.edu/
http://www.coolmath4kids.com/
http://www.funbrain.com/
http://www.harcourtschool.com/activity/thats_a_fact/english_K_3.html
http://www.bgfl.org/custom/resources_ftp/client_ftp/ks2/maths/fractions/level4.htm
Reading:
http://www.spellingcity.com/
http://www.storylineonline.net/
http://www.abcya.com/third_grade_computers.htm
www.scootpad.com
http://www.wordsmyth.net/
http://www.internet4classrooms.com/skills_3rd_original.htm
http://classroom.jc-schools.net/basic/la-grammar.html
Science:
http://kids.nationalgeographic.com/kids/