Mendeley can serve as a heavy-duty citation manager, suitable for long-term research projects and is also a great tool for organizing a robust collection of PDFs. Mendeley also provides users with a rich social networking experience (similar to that of Last.fm). Mendeley allows you to make your work discoverable to other researchers. You can also easily share resources with collaborators using Mendeley groups.
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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. Mendeley is a great organization tool for those who have accumulated a large number of PDF files during the research process. Using Mendeley, you can extract metadata from your PDFs which will allow you to index and organize your collection for easy searching and citing. Mendeley is a desktop application but also has an online component so you'll need to download the software onto your computer in order to get started. Mendeley also offers users a mobile option called Mendeley - Reference Manager (Lite) for iPhone, iPod Touch, and iPad.
(from Mendeley's Features page)