Free-Ebooks – A site with many free ebook titles with varying levels of registration. The free level of service currently allows five downloads per month.
Project Gutenburg – A site with a deep title list, but most publications are in plain text (.txt) or .html formats.
Free Ebooks – A site that brings both authors and readers into the world of free ebooks, downloadable in various popular formats and categories. No registration required and frequently updated.
Bookyards – A site that carries a broad range of free .pdf public domain works with a Canadian bent.
Open Culture – A site that not only carries a pretty good list of free ebooks, but also films and educational materials (including textbooks). They even offer free courses.
Bookboon – Free ebooks and textbooks, no registration required.
ManyBooks – More than 29,000 titles, all free. Most with cover art and all compatible with popular eReader programs.