Zotero is a bibliographic management tool that works with Firefox, Chrome, & Safari, as well as having a desktop program. It recognizes and extracts bibliographic information from websites, library catalogs, and databases, enabling you to easily save, organize, and create bibliographies of the sources you collect. Zotero also has several features that enable you to enhance the citations you collect, including automatic capture of webpages, storage of PDF files, images, and links, as well as note taking and search capabilities. Zotero is powerful, easy to use, and free.
UPDATE September 2017
Zotero is changing: Starting with the release of Zotero 5.0, Zotero for Firefox and Zotero Standalone have been replaced by a single standalone application for users of all browsers. A new Zotero Connector for Firefox, similar to the extensions available for Chrome and Safari, allows saving to the Zotero application or zotero.org in a single click.
For more information, visit the Zotero website.