Originally Posted by Wikipedia

SparkNotes, originally part of a website called The Spark, is a company started by Sam Yagan, Max Krohn, Chris Coyne, and Eli Bolotin in 1999 providing study guides for literature, poetry, history, film, and philosophy. Later, other subjects including, but not limited to math, health, physics, biology, chemistry, economics and sociology have been added. There is no charge for use; the site uses advertising for revenue.

Originally Posted by Wikipedia

CliffsNotes (formerly Cliffs Notes, originally Cliff's Notes and often, erroneously, CliffNotes) are a series of student study guides available primarily in the United States. The guides present and explain literary and other works in pamphlet form or online. Detractors of the study guides claim they let students bypass reading the assigned literature. In contrast, the company claims to promote the reading of the original work, and does not view the study guides as a substitute to the reading.

Originally Posted by Wikipedia

A free online collection of more than 2,300 micro lectures via video tutorials stored on YouTube teaching mathematics, history, finance, physics, chemistry, biology, astronomy, and economics.

Originally Posted by JohnK

I'd like to suggest http://www.khanacademy.org/ be added to the list. Very useful.