Cybraryman.com This site has access to over 20,000 websites and is very thorough - it's an excellent resource for all grade levels.
Khan Academy This site is best known for math but is also good for science and has added art history. It contains videos for all levels.
Math.com Get help with math at all levels even provides homework help
Marcaria- Internet Resources on Citation
Spellingcity.com Great site for spelling and vocabulary primarily for elementary
Studyladder.com A comprehensive site of all disciplines and levels
WolframAlpha.com This is a fact-finding site, probably best for junior high/high school