Public domain works are books, screenplays, art, and movies whose copyrights have expired. These items are typically 70-75 years old or older, but it isn’t that simple. Some works, like movies, may have newer versions or soundtracks. The older version of the movie may be in the public domain, but the updated work is not. Also, copyrights must be renewed periodically. If the copyright is not renewed its copyright might enter the public domain before the time is up. An interesting case of copyright is the Night of the Living Dead. The makers of the movie forgot to put the copyright mark on the film, so it has always been in the public domain. It gets even trickier with characters. Mickey Mouse will soon be in the public domain. Not the familiar one we know today, but the original that debuted in 1928, unless Disney’s lawyers successfully sue to retain the rights.
Here are some works that are in the public domain and some links where you can find some great material yourself.