Mendeley is a PDF and citation manager that indexes and organizes your research library into a digital bibliography. The program includes a collaboration component that allows you to share citations with other researchers. Mendeley is both a desktop application and a web-based application.
Mendeley is a "freemium" service, offering a free basic plan and a premium service for an additional fee. The Mendeley Free Edition provides 2GB of personal storage space, 100 MB of shared library space, up to 25 collaborators, and up to 5 private groups. Various premium plans are available from the Mendeley Upgrade page (Mendeley account login required).