Copyscape (http://www.copyscape.com) is a service that finds copies of your content on the web. Simply type in the URL of the page of your original content and it goes to work, finding matching content in seconds. If you find pages that are plagiarising your work, there are also helpful articles and links to legal sites to guide you in what to do.
Update December 2007: For more information on what to do if your website, blog postings. etc. are being copied, see
[This article was first published in the September 2004 CyberText newsletter; links last checked August 2011]