SparkNotes

http://www.sparknotes.com

SparkNotes, originally part of a website called The Spark, is a company started by Harvard students Sam Yagan, Max Krohn, Chris Coyne, and Eli Bolotin in 1999 that originally provided study guides for literature, poetry, history, film, and philosophy. Later, SparkNotes expanded to provide study guides for a number of other subjects, including math, health, physics, biology, chemistry, economics and sociology. SparkNotes does not charge users, but instead uses advertising for revenue.

http://en.wikipedia.org/wiki/SparkNotes

At SparkNotes, we give you multiple ways to study: online, on your eReader, and with good old books. Here's what we offer:
SparkNotes Guides: Our guides contain thorough summaries and insightful critical analyses. We offer more than 500 guides for English literature and Shakespeare, and a vast number of guides for history, math, biology, and other subjects. Our most popular guides now include quick quizzes, so you can test your retention before the test.
No Fear Shakespeare: No Fear Shakespeare provides side-by-side translations of Shakespeare into plain English. No Fear Shakespeare is available online and in book form.
SparkCollege: Our college bloggers have the inside scoop on getting into school and paying for it once you're there. Need to read up on the schools before you apply? Our college profiles give you immediate online access to key info.
Test Prep: We provide books and online content to prepare you for the SAT, ACT, AP Subject Tests, and GRE. Our test prep blog breaks down the SAT and ACT into manageable portions. Follow us on Twitter @SparkNotes and @SparkNotesSAT for a new SAT word and definition each day.
Flashcards: Our online flashcards will help you study for biology, history, literature, and SAT and ACT vocabulary.
Miss Marm: Miss Marm, our writing expert, answers your questions, hosts fiction contests, and explains tricky grammar rules on her blog. She also tweets writing tips and news @MissMarm.
SparkTests: Are you in love? When will you die? Exactly how lazy are you? Learn amazing facts about yourself with our collection of SparkTests.
SparkLife: Our blog, SparkLife, will make you ALOL. It helps explain school and life with posts about homecoming, lunch, teachers, and procrastinating techniques; thoughtful advice on stupid boy/girlfriends; quizzes and polls; original articles from high school students; and blogs on Twilight, Lord of the Flies, and other popular books.
SparkNotes on facebook and Twitter: Become a fan of SparkNotes on facebook and follow us on Twitter. We'll post updates, give you new homework help, and ask your opinion on which academic material to tackle next.
We're always evolving and creating new content, and we welcome your input and requests!