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.
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.
Write & Cite
General citation guides
American Psychological Association guide (used in the social sciences, and sometimes academic papers regardless of subject)