I’m assuming everyone already knows about Gutenberg and Bartleby.
- Feedbooks (http://feedbooks.com/)has a large selection of public domain and other free-licensed works available. The default format is epub, but a couple of other formats (PDF, Mobipocket/Kindle) are also available. Feedbook is also a place to self-publish and host your own free ebooks. Publishing requires registration, but downloads don’t.
- Freelance Writing (http://www.freelancewriting.com/ebooks-for-writers.php) links to a large number of free ebooks for writers. Most, if not all, are external links.
- Fiction Factor (http://www.fictionfactor.com/writersfreebies.html) is similar to the above, but oriented toward fiction writers. A mix of on-site and external links.
- The U.S. Government (http://www.america.gov/publications/books.html) has a surprising variety of free e-publications available. They’re produced with your tax dollars (if you’re a U.S. taxpayer), so you might as well take a look.