Mendeley is a reference manager like the other tools featured on this page with some added features. As a reference manager it allows you to create citation and bibliographies in using a variety of document creation platforms including Microsoft Word, OpenOffice and LaTeX. In addition it provides a variety of other features including the ability to read and annotate pdfs within software, collect and organize pdfs, share your papers notes, and discover new papers, people to collaborate with and groups.
There are limits of the number of members to a group in a free account
Must have web account in addition to desktop software to work on multiple computers