Founded in 1971 by Michael S. Hart, it is the oldest digital library. Most of the items in its collection are the full texts ofpublic domain books. The project tries to make these as free as possible, in long-lasting, open formats that can be used on almost any computer. As of December 2009, Project Gutenberg claimed over 32,000 items in its collection.

If you have a lot of classics in your “To Be Read” file, you can bet the text if available from Project Gutenberg since most of these books are older and have passed into the public domain.  Project Gutenberg is careful to verify the status of its ebooks according to U.S. copyright law. Material is added to the Project Gutenberg archive only after it has received a copyright clearance, and records of these clearances are saved for future reference. Unlike some other digital library projects, Project Gutenberg does not claim new copyright on titles it publishes. Instead, it encourages their free reproduction and distribution.  Most books in the Project Gutenberg collection are distributed as public domain under U.S. copyright law.

Although still considered experimental by PG, most of the books are now available already formatted in EPUB and MOBI formats.  That means you Sony, Nook, and Kindle users can read these on your device with no messy conversions required.

PG indexes all of their books and for data-hounds, this information is downloadable and searchable.  Further, you can download ISO files (CD images) containing all books on PG, or categorized by genre (like Sci-Fi).

If you are really ambitious, you can read most of the Harvard Classics (Five Foot Shelf) on PG.

I highly recommend reading some of the articles listed on the PG About Page.