Project Gutenberg (PG) is a volunteer effort to digitize and archive cultural works, to "encourage the creation and distribution of eBooks". It was founded in 1971 by Michael S. Hart and is the oldest digital library. Most of the items in its collection are the full texts of public domain books. The project tries to make these as free as possible, in long-lasting, open formats that can be used on almost any computer. , Project Gutenberg reached 50,000 items in its collection. 

The releases are available in plain text but, wherever possible, other formats are included, such as HTML, PDF, EPUB, MOBI, and Plucker. Most releases are in the English language, but many non-English works are also available. There are multiple affiliated projects that are providing additional content, including regional and language-specific works. Project Gutenberg is also closely affiliated with Distributed Proofreaders, an Internet-based community for proofreading scanned texts. 

Project Gutenberg was started by Michael Hart in 1971 with the digitization of the United States Declaration of Independence. Hart, a student at the University of Illinois, obtained access to a Xerox Sigma V mainframe computer in the university's Materials Research Lab. Through friendly operators, he received an account with a virtually unlimited amount of computer time; its value at that time has since been variously estimated at $100,000 or $100,000,000. Hart has said he wanted to "give back" this gift by doing something that could be considered to be of great value. His initial goal was to make the 10,000 most consulted books available to the public at little or no charge, and to do so by the end of the 20th century.
What is a format used?
 plain text