Intellectual property passes into the Public Domain (no longer under copyright) depending on several factors including the copyright law at the time of publication, age of the work, lifespan of the author, and others. For specific information about copyright term, check out this handy guide from Cornell University.
Project Gutenberg is quite possibly the greatest archive of public domain material ever placed online, and it keeps getting bigger. Gutenberg contains classic literature (including those Kate Chopin stories that are so popular in our ENGL courses), works of non-fiction, and more - all in the public domain and free to use.
The Library of Congress stands beside Gutenberg as the greatest archive on Earth. Part of the LC collections is a digital archive of public domain images. These images are free to use and reuse.
Did you know that most U.S. federal documents are in the public domain? Here's a handy database of government publications from the GPO including reports, periodicals, and ebooks.