Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Diigo - Diigo is a powerful research tool and a knowledge-sharing community
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
WorldBrains Memex - Bookmarking for the power users of the web. A privacy focused extension to annotate, search and organize what you've seen online.
EasyBib - Automatic works cited and bibliography formatting for MLA, APA and Chicago/Turabian citation styles.
BibGuru - BibGuru is a bibliography and citation generator for students.