Mendeley is a web-based, free-to-everyone (up to 2 GB of online storage) citaiton 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.
Many other citation management tools are available, many for free. 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.
Acknowledging Sources: Citation
This tutorial helps students understand the necessity of citing properly to avoid accidental plagiarism.