SparkNotes - study guides

SparkNotes, originally part of a website called The Spark, is a company that was started by Harvard students 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 were added.
The content on the site is free.
Check down the left hand side of the home page, for the headings:
Top Video SparkNotes
Top SparkNotes
Top No Fear Shakespeare
Top No Fear Literature
Test Prep
http://www.sparknotes.com/

SparkNotes are also available in book form.