If you've never heard about Project Gutenberg then it's about time you did. This project is a volunteer effort to digitize and archive cultural works, or more precisely “to encourage the creation and distribution of eBooks”. Founded in 1971 by Michael S. Hart, it is the oldest digital library filled with mostly texts of public domain books. As of November 2010 Project Gutenberg claimed over 34,000 items in its collection.
With the goal “to provide as many e-books in as many formats as possible for the entire world to read in as many languages as possible.” most releases are in the English language, but many non-English works can also be found on the site, and they are all available in several formats such as plain text, html, epub, among others.
Project Gutenberg is also careful to verify the status of its eBooks according to U.S. copyright law and most books in the collection are distributed as public domain under U.S. copyright law. That said, take a look around on the site. You'll find a number of classics there that are worth being read or re-read.
Charles Dickens? William Shakespeare? Oscar Wilde? Jane Austen?
You name it. And now, go ahead, and have fun reading.