Good online study guides - the kind that aren't trying to sell you pre-packaged essays every five minutes - are few and far between. Here are a few of my favorites:

• SparkNotes - Free Online Study Guides: Literature study guides, math study guides, astronomy study guides - you name it, you'll probably find it here.
• FreeBookNotes.com: "A comprehensive guide to free book notes, free book summaries, literature notes, and study guides (like "cliff notes") for over 1600 books, plays, and poems."
• Literature Study Guides: Includes the complete works of Shakespeare here - a very extensive site with new study guides added every week.
• Understanding Mathematics - Math Study Guide: A University of Utah professor has written a stellar guide on figuring out the foundation of all mathematics; this is very helpful.
• Free Online Literature and Study Guides: "The Bibliomania Study Guides have been produced by the Bibliomania Creative Team, a group of final year and postgraduate literature students from the top UK and American universities."

Related: In addition to online study guides, you can find a lot of useful research information on the Web by utilizing research and reference experts, checking out the best reference sites, or searching on the Deep Web.