Live help, study aids, general facts and specific subjects as well as tips from the pros on how to maximize study strategies.
Fact Monster - Lots of subjects and reference information as well as pop culture
Infoplease Homework Center- General subjects and reference information
Math: the World of Math Online- Every math subject plus calculators, worksheets and games
Purdue OWL (Online Writing Lab)- general writing and research resources
Khan Academy - tutorials, videos, and assessments for most subjects as well as test prep