Zotero is a free, open tool that helps you gather, organize, share and use citations to articles, books, book chapters and more. It is available as a plug-in for the Firefox browser or as a standalone program that can be used with many browsers, such as Google Chrome and Safari. It works with the Macintosh, Windows and Linux operating systems.
Since it works within your browser, you can add references as you're using the web. You can add PDFs, full text, audio and video. Zotero creates an automatic index to your citations so you can search your personal database and find references to use.
Register for an account to get storage space and to use your database from any web browser.
As their web site says: "Zotero is a project of the Roy Rosenzweig Center for History and New Media, and was initially funded by the Andrew W. Mellon Foundation, the Institute of Museum and Library Services, and the Alfred P. Sloan Foundation." The outlook is for it to remain free and independent for the foreseeable future.