Here are some of the most substantial online libraries either specializing in free online classics translated into English, or offering an even more comprehensive collection that includes the classics. THOUSANDS OF BOOKS for your reading pleasure.
 

Classics

Online Medieval and Classical Library (OMACL)
http://sunsite.berkeley.edu/OMACL/
A collection of English translations of hundreds of important Classical and Medieval literary works.

Internet Classics Archive (Tech Classics )
http://classics.mit.edu/
Several hundred translated texts from 17 different Greek and Latin authors in HTML  format. 

Diotima Anthology of Translated Materials
http://www.uky.edu/ArtsSciences/Classics/anthology.html

Electronic Resources for Classicists
http://www.tlg.uci.edu/~tlg/index/about.html

Orb Library
http://orb.rhodes.edu/library.html

Library of Congress Greek and Roman Classics
http://lcweb.loc.gov/global/classics/clastexts.html

Perseus Texts
http://www.perseus.tufts.edu/Texts/dump_TOC.html
English translations, 3 dozens classical authors. Part of Tufts enormous
searchable database of the ancient world.

Link Pages on the Classics

Myth and Legends (Princeton)
http://pubpages.unh.edu/~cbsiren/myth.html

Internet Resources for Classicists
http://www.sms-va.com/mdl-indx/internet.htm

Voice of the Shuttle Classical Resources
http://www.showgate.com/medea/grklink.html

Classics Links at U.Michigan
http://rome.classics.lsa.umich.edu/welcome.html

Diotima
http://www.uky.edu/ArtsSciences/Classics/gender.html

Search Argos: Ancient/Medieval Search Engine
http://argos.evansville.edu/
 

Project Gutenberg http://www.gutenberg.net/_titles/_A_index.html
Books whose copyright has expired, converted into text files.

Google Online Books http://books.google.com/books?q=
The largest online collection of in-print and out of print classics of all kinds. Many are scanned copies of early editions. Continually expanding.

Electronic Text Center
http://etext.lib.virginia.edu/english.html
Electronic Text library at the University of Virginia, most but not all accessible to the general public.

English Server
http://eserver.org/
Carnegie-Mellon's online collection of 20,000 texts.

Internet Public Library Online Texts Collection
http://www.ipl.org/reading/books/
Over 7700 online titles.

Oxford Text Archive
http://ota.ahds.ac.uk/index.html
Several thousands mostly free electronic texts in several languages.

Carrie Electronic Text Library
http://kuhttp.cc.ukans.edu/carrie/stacks/stacks_main.html

Athena: The Internet  Library
http://un2sg4.unige.ch/athena/html/authors.html
Links to electronic and downloadable versions of texts arranged in alphabetical order by author and title. Links to other e-text archives. 

More Link Pages to Electronic Texts
Electronic Text services
http://www.columbia.edu/cu/libraries/indiv/ets/offsite.html

Etext Links
http://www.tlg.uci.edu/~tlg/index/texts.html

Electronic Journals
http://www.leeds.ac.uk/library/ejourn/

Alex Electronic Texts: English Literature and Philosophy
http://sunsite.berkeley.edu/alex/

Project Bartleby http://www.cc.columbia.edu/acis/bartleby/

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