EndNote is a reference management software that allows you to search databases for citations, import citations, store and organize your citations, create bibliographies and share with up to 14 others (all users must pay for the software). There is a free, basic version of EndNote available that has limited storage and does not allow sharing files with colleagues.
Refworks is a reference management software that is designed to help researchers gather, manage, store and share all types of information, as well as generate citations and bibliographies. All users must pay for access to the software. There does not appear to be a free version of this software.
A largely free reference management software that allows notations in PDF files. Captures, stores, imports, exports and organizes citations. One free collaborating group of 3 allowed, more with paid subscription.
A completely free, (up to 300 MB) open-source reference management software developed and owned by George Mason University. Collects, organizes, cites and shares research sources. Allows unlimited collaboration.
Further Information on Citation Management Software
Wikipedia has a nice table comparing numerous citation/reference manager software programs