Mendeley is a free* research tool that assists at every stage of the research process, from gathering research and annotating PDFs to organizing citations and creating bibliographies. Mendeley attempts to unite many different tools (EndNote, RefWorks, Zotero, Academia.edu, LinkedIn, SelectedWorks, CiteULike, Papers, JabRef, etc.) into one, and serves as three major tools: Reference Manager, PDF Organizer and Social Network.
*Mendeley is a "freemium" service, which means the basic account is free, but you can access premium features and additional storage for a monthly fee. You can find more information on individual and team plans on the Mendeley Upgrade page.