Project Gutenberg is a volunteer run and operated virtual library that supplies readers with eBooks of many out of copyright works of fiction and nonfiction. Founded in 1971, it claims the title of world's oldest digital library.
Open Culture is a site dedicated to supplying the world with public domain books, audio books, movies and other educational materials.
While first and foremost a bibliophile social group, Good Reads has other fun resources for readers including this library of free ebooks (including the semi-authorized/endorsed sequels to the Harry Potter series).
The Book Depository is primarily an online book store; however, they also have free eBooks available at this link.
Kobo, like the Book Depository, is mostly a for-profit book store, but also offers over one million free eBook downloads.
Books Should Be Free houses over 3,000 audiobooks sourced from public domain works.
Librivox, the little sister to Project Gutenberg, enlists the help of hundreds of volunteers to render public domain books, speeches, textbooks, articles, and any other written work into audiobook form. Quality varies, but there are over 8,000 titles to choose from.
Podiobooks is a "creative commons" site that offers audiobooks for free, but asks those that can afford it to donate towards the works they enjoyed to reward and compensate the authors and recorders.
Pottermore is the home to all things Harry Potter. Written and curated by J. K. Rowling,
it houses encyclopedic articles on everything found in the Harry Potter universe including unpublished HP short stories.