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.
Public Domain(?):
Stanford Copyright Renewal Database - Allows you to 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.
Fair Use:
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.
Center for Media and Social Impact: Fair Use Library - A must.
Creative Commons:
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.
EDIT (5/21/23): Fixed some broken links.