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).
The Mendeley Institutional Edition provides 100 GB of personal storage, 100 GB of shared library space, up to 100 collaborators, and an unlimited number of private groups. Stanford Libraries currently provides the Mendeley Institutional Edition but is exploring options during AY2018-2019. To initiate access to the Institutional Edition, click the "join this group" button on the Stanford Mendeley Group page.