Project Gutenberg. From their welcome page blurb:
Project Gutenberg offers over 42,000 free ebooks: choose among free epub books, free kindle books, download them or read them online.
We carry high quality ebooks: All our ebooks were previously published by bona fide publishers. We digitized and diligently proofread them with the help of thousands of volunteers.
There is a sister site, Project Gutenberg Austrailia that has some advantages over the original PG site. Austrailia's copyright laws are much more sane than those in the U.S., so they are able to publish some books that are in the public domain there that are still considered to be under copyright here. I recently downloaded and read through the entire Lensman series by E.E.Smith. That's some seriously classic SF.
Beat me to it. Thanx for the Assuie link. I hadn’t known of that project.