Zotero is a Firefox extension designed for collecting, managing, and citing research sources.
It's different from commercial citation tools like EndNote, BibTex, RefWorks, etc., in that it works from directly within the browser itself. And it's free. But you have to use it with the Firefox browser.
A review of Zotero can be found in this blog, maintained by reference librarians at Duke University (USA). If you have time, check out this webcast dedicated to Zotero at 2:00 PM EST on Thursday, January 10, 2008.