You can find free books for your e-readers using the following links:
Project Gutenberg is the original website for free ebooks. They have over 39,000 ebooks available in epub format. These are mostly older books that were either published before copyright laws or their copyrights have expired.
http://manybooks.net/ has over 29,000 books available for free in various ereader formats.
http://openlibrary.org/ has over 1,000,000 free ebooks.
http://www.barnesandnoble.com/u/Free-eBooks/379003078/. This link takes you straight to B&N’s page for ebooks that are currently free.
http://www.fictionwise.com/ is a free ebook listing that is also created by Barnes & Noble.
http://kindlenationdaily.com/ lists all the free ebooks listed on Amazon. They also have a reduced-price feature every day.