Mendeley is a web-based, free-to-everyone (up to 2 GB of online storage) citation management tool supported by Mac, Windows, and Linux. It is also available as a mobile app. For more information, visit the Mendeley website.
Zotero is a free citation management application (supported by Mac, Windows, and Linux). For more information, visit the Zotero website.
The University of Wisconsin-Madison has produced a very nice comparison chart of the popular citation management systems. For a very thorough comparison of many free and paid systems, check out Wikipedia.