According to the U.S. Copyright Office:

"The public domain is not a place. A work of authorship is in the 'public domain' if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner." 

Project Gutenberg

6000+ full-text electronic books mostly in the public domain.

Project Gutenberg Audio Books
Human-read and computer-generated audio books in the public domain.

Hathi Trust Digital Library

Digitized collections of some of the nation’s great research libraries available for all.

 

Online Books Page
25,00 free books on the web.

Prelinger Archive

2,000 videos from hundreds of important US corporations, nonprofit organizations, trade associations, community and interest groups, and educational institutions

LibriVox
Recorded books in the public domain.

Public Domain Music
Lists of public domain music.

Musopen
Recorded public domain music.