Zotero (pronounced "zoh-TAIR-oh") is a Firefox add-on that collects, manages, and cites research sources. It's easy to use, accessible within your web browser where you do your work, and best of all it's free. Zotero allows you to attach PDFs, notes and images to your citations, organize them into collections for different projects, and create bibliographies using Word or OpenOffice. It is easier to use than EndNote or RefWorks and surprisingly powerful.
The easiest method to start using Zotero is from the Firefox web browser. Download Zotero directly from the Zotero homepage by clicking on the red button in the top righthand corner.
Since it's a Firefox plugin, it automatically updates itself periodically to work with new online sources and new bibliographic styles.