Project Gutenberg was the first website to offer free ebooks and has been around since 1971. It is a very large collection of books in the public domain that have been converted to digital format.

Open Library is a project of the Internet Archive and is an online catalog of over 20 million books. In addition, they have for download over 1.7 million public domain ebooks and some public libraries also list their ebooks with them.

Open Culture is a website with a curated list of free ebooks, audio books, and videos.  They offer fiction as well as nonfiction, online courses, movies, and language learning resources.

ManyBooks was established in 2004 and many of the early ebooks are from the Project Gutenberg archives. They also have newer books from Amazon or other online book sellers that are free or for sale.

Loyal Books has a large collection of classic fiction and nonfiction ebooks and audio books.  Most of their collection is from Project Gutenberg and LibriVox.

Google eBookstore – Google sells ebooks through Google Play and they always have some titles available for free.  It is an easy way to get ebooks to read on your mobile phone.

Freebook Sifter lists free ebooks from Barnes and Noble, Project Gutenberg, Kobo, and Amazon. They have hundreds of thousands of free ebooks listed.

E-Books Directory is a free web resource which contains links to free downloadable e-books, technical papers, documents, as well as user-contributed content.  It has more than 10,000 ebooks.