Zotero is a free reference management program that interacts seamlessly with online resources: when it senses you are viewing a book, article, or other object on the web, it can automatically extract and save complete bibliographic references.
A Web-based bibliography and citation manager that enables users to create their own personal database of citations by importing references from the library catalog, library databases, and elsewhere.
Mendeley is a free reference manager and academic social network. Make your own fully-searchable bibliography, cite as you write, and read and annotate your PDFs on any device.