Mendeley is a citation management desktop client based primarily around the cultivation of PDFs, but can also be used for managing and sharing research papers, discovering research data and collaborating online. You can import PDFs, highlight and annotate them, get readership statistics, and even choose a folder on your computer for Mendeley to "watch" for new content. Mendeley also gives you the opportunity to search across papers cultivated by other Mendeley user. Mendeley requires the user to store all data on its servers. Upon registration, Mendeley provides the user with 1000 MB of free space, which is upgradeable at a cost.
You can download Mendeley here.
This list of links will help you use and manage the tool: