The Copyright Clearance Center is a U.S. company that serves as a go-between for copyright holders and those who wish to use their materials. The CCC provides useful information on copyright and copyright compliance, including:
This excellent site provides not only information on how to stay compliant when using others' work, but also information on protecting your own work and effectively managing your copyrighted materials.
A brief, humorous look at changes in length of time copyright is protected. (Note: includes a very brief instance of language that some may find offensive.)