All subject areas:
www.funbrain.com
www.internet4classrooms

Science:
www.discovery.com
www.yahooligans.com (Click on the science tab)

Math:
www.aplusmath.com
www.aaamath.com

Spelling:
www.puzzlemaker.com (This is a great site to practice spelling words. You can have your child type in their words and print a puzzle or word find. Then they complete it and it reinforces great spelling.)

Reading:
www.scholastic.com
www.time4kids.com