Daily visits:
Project Gutenberg:
http://www.gutenberg.org/catalog/
Books in the public domain in the United States
Project Gutenberg Canada:
http://gutenberg.ca/#h2newreleases
Books in the public domain in Canada
(Canada uses Life+50 years as the period for copyright, so there are works that are in the public domain in Canada but not the US.)
Project Gutenberg Australia:
http://gutenberg.net.au/plusfifty.html
Books in the public domain in Australia. (It used to have a page of works in the public domain in Australia, but not elsewhere. AU has changed its laws on copyright to correspond to the Life+70 years practice in the US and elsewhere, but works already considered in the public domain were not retroactively removed from public domain status, so tehre tre a variety of things available there but not elsewhere.))
Munseys:
http://www.munseys.com
Formerly known as Blackmask. Mostly works sourced from Project Gutenberg, but a fair bit of stuff not available elsewhere. The proprietor is a fan of "noir" and "hard boiled" mysteries, and old pulp magazine stuff that no one has gotten around to adding to PG. Books are offered in a variety of popular formats, produced by scripted conversion from source.
ManyBooks.net:
http://manybooks.net
Mostly material sourced from Project Gutenberg, but with a healthy offering of Creative Commons licensed works as well. The proprietor pushes ManyBooks as a distribution site for such things. Books are offered in a variety of popular formats, produced by scripted conversion from source.
Periodic visits:
The Internet Sacred Text Archive:
http://www.sacred-texts.com/
An enormous quantity of material related to spirituality, religion, philosophy, and culture.
The Online Books Page:
http://onlinebooks.library.upenn.edu/
An online library offered by the University of Pennsylvania
Perseus Digital Library:
http://www.perseus.tufts.edu/hopper/
A collection of works oriented to classics and the antiquities from Tufts University.
The Baen Free Library:
http://www.baen.com/library
A collection of freely available SF and fantasy novels by authors in Baen's stable, in a variety of formats. A number of books in the Free Library are not listed on the Baen site, but appeared in bound-in CDs in selected Baen hardcovers. A complete set of the CDs in Zip and ISO image format, is available at The Fifth Imperium:
http://baencd.thefifthimperium.com
The On-line Library of Liberty
http://oll.libertyfund.org/
A variety of works on politics and economics
There's more, but this is a start.
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Dennis