Some helpful links: U.S. Copyright Office Copyright is a kind of intellectual property monopoly. And if it was intellectual property Monopoly, this site would be "Go." Translation: Start here.
How to Register a Work This site takes you to eCO, where you can file a copyright registration for your work through the Copyright Office online system.
Stanford Copyright Renewal Database Lets you search for whether a work is still under copyright or not.
Copyright Term and the Public Domain in the United States A chart to help you understand the labyrinthine laws regarding when a work will fall into the public domain. The chart is available as a PDF.
Documentary Filmmakers’ Statement of Best Practices in Fair Use If you are a documentary maker you should know this up and down.
Code of Best Practices in Fair Use for Online Video Like the Documentary Best Practices, this is something to know and learn.
Creative Commons. Where to go if you want to give it away, legally speaking.
Resource pages and other links:
Stanford University Libraries: Copyright & Fair Use: Charts and Tools: A great page of links.
Cornell University Copyright Information Center: More great links.
EDIT (7/9/08): This post was accidentally deleted. I think I've restored it pretty completely, and added some more links in the process.
EDIT (9/29/16): Fixed some broken links.