Testing & Education Reference Center (6 - 12)
Find practice tests for the SATs, ACTs, regular math and English tests, scholarship searches, and help for every subject. You can even set the date for your next test and get a study schedule created to help.
Khan Academy (K - 12)
Great Videos on every math concept that make it easy to understand and fun to learn. If you're having trouble wrapping your head around a certain problem, look here first.
Purdue Online Writing Lab (7 - 12)
Get some great tips to help with researching papers or essays. There's even a section on how to conquer "writer's block", that thing that prevents you from getting words from your brain into a computer the night before an assignment is due.
Student Resources in Context (9 - 12)
Look up a topic and get a quick collection of articles, videos, photos, and basic facts on a huge range of topics. This is like Wikipedia, but more appropriate for school projects because it has primary sources cited and ready to use.
Literature Resource Center (6 - 12)
Check out biographies, bibliographies and critical analysis of authors from every age and literary discipline. Great starting point for book reports and biographies.