Archive.org
The Internet Archive is a Non-Profit library whose mission started as an attempt to archive the internet itself. There current holdings include
- 279 billion web pages
Project Gutenberg
Project Gutenberg makes over 53,000 books within the public domain, available in multiple formats. Books can be read online or downloaded
Open Library
Open Library is an open, editable library catalog, building towards a web page for every book ever published.
DPLA
The Digital Public Library of America is a forward-looking effort to connect diverse library resources and make them useful and extendable to patrons and institutions.