Amazon makes available downloadable copies of classics and books that are out of copyright (mostly pre-1923 titles).
Barnes and Noble Search for ‘free ebooks’ to browse by subject/genre.
Complete Works of William Shakespeare Viewable online, but not downloadable.
Forgotten Books Thousands of free ebooks in PDF format (also a print bookseller). Focus on historical writings (e.g. classical fiction, philosophy, science, religion, folklore, mythology and sacred texts).
Google Books use “Advanced Book Search” and search “full view only”.
Harlequin Allows up to 18 free downloads.
Project Gutenberg over 38,000 free ebooks.