Mendeley's online citation management software is free (1GB of storage) and you may purchase subscriptions to gain more storage space.
Mendeley adds social networking to citation management. Join a Public Group on Mendeley to share citations and have citations suggested based on similar keywords, authors, or journals; or start a Private Group to share full-text articles with up to 10 people (free users may join 5 Private Groups and an unlimited number of Public Groups; payment plans can increase the Private Groups available).
Download Mendeley to your desktop and add PDFs, then create bibliographies and formatted papers in Microsoft Word, LaTex, or Open Office.
Use Mendeley online to access groups and sharing and add citations from webpages.
Your library is synched across the online and desktop platforms, so you can access your library anywhere.
Signing up for and setting up a Mendeley account.
Importing citations from PubMed, from a PDF, and using Mendeley's crowd-sourced catalog.