Zotero is a Firefox 2.X add on that is currently free. Zotero is compatible with Windows, Mac and Linux. If you spend countless hours on the internet each day then Zotero is a very valuable resource. You can collect and categorize data, as well as tag, archive and link to it. It allows you to manage and cite your research sources easily and quickly because it works within the browser itself. Zotero is great for storage of all your PDF files as well as images and complete web pages.