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).
Documentation and tutorials:
"I received an email that I haven't signed in to Mendeley from my institution in the last 11 months and that my alumni access to premium benefits associated with Mendeley Institutional Edition will expire in a month. I am still at Stanford, and I am not an alum. What can I do?"
To maintain your premium benefits, log in to your Mendeley account while you are on campus. If you will not be back on campus for over a month, then access Mendeley Online using EZproxy (SUNET ID required), and then log in to your Mendeley account.
The Mendeley Support Center provides additional information about this 11-month warning email.