Mendeley is a free reference manager and academic social network that can help you organize your research, collaborate with others online, and discover the latest research. Advantages of Mendeley include automatically generate bibliographies, collaborate easily with other researchers online, easily import papers from other research software, find relevant papers based on what you're reading, and access your papers from anywhere online.
Zotero is a free and open-source reference management software to manage bibliographic data and related research materials. This tool collects all your research in a single, searchable interface. You can add PDFs, images, audio and video files, snapshots of web pages, and really anything else. Zetero automatically indexes the full-text content of your library.