Reference management tools are designed to help researchers easily gather, manage, store, and share all types of information and generate citations and bibliographies.
Mendeley is a free reference manager and academic social network that allows users:
More information about Mendeley: http://community.mendeley.com/guides
Zotero is a free, open-source research management tool that allows users to:
More information about Zotero: https://www.zotero.org/support/quick_start_guide