Fun Brain: great activities for reading and math http://www.funbrain.com/
Starfall: reading practice for all levels http://www.starfall.com/
Spelling City: a great way to practice those weekly spelling words http://www.spellingcity.com/index.php?option=com_content&task=view&id=1103&Itemid=24
Scholastic: everyone's favorite reading resource http://www.scholastic.com/kids/stacks/
A Plus Math: math games http://www.aplusmath.com/Games/index.html
National Library of Virtual Manipulatives: physical representations of math concepts at many different levels http://nlvm.usu.edu/en/nav/vlibrary.html
National Geographic Kids: self-explanatory! http://kids.nationalgeographic.com/
Time for Kids: a great way to keep up with current events http://www.timeforkids.com/TFK/
Reading is Fundamental: fantastic stories for reading practice http://www.rif.org/readingplanet/content/read_aloud_stories.mspx
Surfing the Net with Kids: games and puzzles in different subject areas http://www.surfnetkids.com/
Songs and Rhymes: songs for students in all grades http://kids.niehs.nih.gov/musicchild.htm
National Library of Virtual Manipulatives http://ww.nlvm.usu.edu
http://www.mikids.com
Lou's Wildlife Sights & Sounds http://www.lousss.com
Tumble Books (City of Orange) http://www.tumblebooks.com/library/asp/home_tumblebooks.asp