Amazon's Free Book Collections and Limited Time Promotional Offers. The Kindle App can be downloaded free here.
Barnes & Noble's Free ebooks- You can also download the Nook app to other devices if you don't have a Nook here.
Bartleby- includes all of the Presidential Innaugural Addresses from George Washington to Barak Obama.
Bookyards- with over 17k books and 384 videos.
Classic Authors.net- not just novels, they also have essays, poems, plays, etc.
Fictionwise- also includes nonfiction and reference sources.
Free-eBooks.Net- good source for nonfiction texts.
Gallica- produced by the Bibliotechque nationale de France. Includes works in English, as well as French.
Internet Archive- includes not only texts, but moving images, and audio files as well.
Many Books.Net- including ebooks in 36 other languages, including Arapaho, Maori, and Tagalog.
Open Library- some books you can have free, and others are available through their lending library.
Project Gutenburg- the first producer of free ebooks.
World Wide School Library- Heavy on the literature, with only a few selections in foreign languages.