The following links represent web sites that I have found useful in researching questions of copyright. I hope you find them useful as well.
US Copyright Office Website: The place to start. You can view the entire text of the US Copyright law online. If that's too much to digest, they also have numerous publications to download, which answer common questions in plain English.
Copyright Term and the Public Domain in the United States: Cornell University provides an excellent, easy-to-read chart of the copyright status of published and unpublished works.
Prosecuting Intellectual Property Crimes: This online manual is provided by the US Department of Justice, with advice on how to seek redress if your intellectual property rights have been violated. This page is not only helpful for its stated purpose; it's also a useful resource for a collagist who uses copyrighted material, and wants to know what sort of penalties might be at risk. Note that this site addresses criminal copyright infringement, not civil (i.e. lawsuits).
Ivan Hoffman: This intellectual property law expert has posted essays on a variety of legal issues. His site is geared more towards the intellectual property concerns of designers and writers working on the Internet. However, there is a great deal of information of use to the collage artist as well.
Lawgirl: Intellectual property lawyer Jodi Sax provides some good basic information on copyright and trademark registration on her site, www.lawgirl.com. Sax's specialty is entertainment law, but again, there's some good information here. She even provides the "lawboard," a bulletin board where she answers legal questions for free.
The Cyberlaw Encyclopedia: This online resource pulls together a wealth of information including links to government publications, recent cases, and articles.
US Patent and Trademark Office Website: It's not copyright, but it is intellectual property. If you plan to include brand names or logos in your work, you might want to check this site.
Benedict.com: This excellent resource uses real-life examples to put copyright law in context. Includes a lawsuit against noted collagist Robert Rauschenberg, which is particularly relevant to the collage artist's concerns.
Know of another site on copyright law that should be included here? Let me know!