EndNote is software that allows you to keep track of your citations, organize them into folders, and create references and bibliographies in your papers.
EndNote is similar to Refworks and Zotero, but it allows you to produce bibliographies in a greater number of citation styles and allows greater customization. It is particularly popular in the sciences.
Unlike these web-based applications, EndNote is not free, and requires download and installation. You can download a 30-day trial version from the Endnote website, or buy it from the University of Illinois WebStore.