Public Domain

Works in the public domain may be reproduced, distributed, modified, adapted, performed or displayed in public without seeking anyone's permission. All works published in the United States before 1923 are in the public domain.
Criteria for public domain are:

• the work was published before there was a copyright law
  
• the work's copyright protection has expired
  
• copyright protection was never acquired
  
• the copyright owner gave the work to the public
  
• the work was never eligible for copyright protection

4Literature - Several thousand books including religious and historical documents, children's fairy tales, and books by famous 19th-century American authors.
Bartleby's - Great books online, reference, verse, fiction, nonfiction.
Cylinder Preservation and Digitization Project
Project Gutenberg - No latest bestsellers or modern computer books here, but plenty of classics from the literary canon.
The Making of America - About 8,500 books on American social history from the ante-bellum period through Reconstruction.
The Online Books Page - About 13,000 books, mostly older titles that are in the public domain.
Creative Commons -- nonprofit organization for sharing creative works
Center for the Study of the Public domain