"Project Gutenberg was the first provider of free electronic books, or eBooks. Michael Hart, founder of Project Gutenberg, invented eBooks in 1971 and his memory continues to inspire the creation of eBooks and related technologies today."
Project Gutenberg has about 60k ebooks, all available without registration, without fees, without any special apps. Includes a mobile version of the site.
The History and Philosophy of Project Gutenberg by Michael Hart is a perfect introduction to the spirit of Project Gutenberg.