Mendeley is a citation management tool, which includes both an online and desktop element. Similar to EndNote, citations can be exported to the system from most databases, and PDFs of articles may be attached to the citations.
Zotero is an open-source web-based citation management tool, which works particularly well for the management of web-based information sources. ZOTERO FUNCTIONS AS A PLUGIN TO THE FIREFOX BROWSER.
The Chicago Manual of Style by 
