Under current copyright law, all work is copyrighted from the moment of creation. Registration and copyright notices are not necessary.
At a certain point, copyrighted works pass automatically into the Public Domain, which means they can be used by anyone for any purpose. Learn more about Public Domain.
Peter Hirtle created a very handy chart you can use to evaluate what material falls into the Public Domain in the United States and thus is no longer subject to the provisions of the Copyright Act:
Some creators choose to grant everyone the right to freely re-distribute and re-use their work. For more information about this, see Copyright Alternatives.