Mendeley is a citation management program that is free to users. There are a minimum number of groups you can create and join on free accounts, but the program provides full functionality to paying and non-paying customers otherwise.
Mendeley is a good option for those who need a more robust platform than RefWorks, but can't afford, or are too intimidated, by EndNote.
You can download a desktop version to manage on your personal computer, but it also provides access to a web interface that syncs to your desktop application. Thus, you can manage, organize and store your citations and pdfs no matter where you are. The desktop version will even crawl your PC to find pdfs and create citations for them in Mendeley.
A plugin for Microsoft Word, Open Office and other document programs allows you to create formatted citations and bibliographies.
System/Browser Support: Windows, Mac, and Linux.
Distinctive Features: In addition to citation management, Mendeley provides social networking opportunities with like-minded researchers, tracks trends, and intergrates with Word, Open Office, Google Docs, and BibTex.
Watch the following ~15min tutorial on setting up a new account and importing references.