Starfall Reading: a great tool for pre-readers, beginning readers, and advanced readers http://www.starfall.com
(Sample link) National Geographic for Kids http://www.nationalgeographic.com/kids/
Kid's Place Houghton Mifflin- for extra math practice correlated directly with the math curriculum http://www.eduplace.com/kids/hmm/
Great information, games, quizzes http://www.brainpop.com
Anything you want to know about any science topic. http://www.discoverychannel.com
Take an AR test https://hosted17.renlearn.com/86760/default.htm
A great website where you can take a practice test over spelling words, and play games using current spelling words. http://www.spellingcity.com