Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Papers by ReadCube - ReadCube Papers is a sumptuous reference manager helping out academic writers to get the most out when it comes to access and discovering more citations with multiple libraries or databases.
Diigo - Diigo is a powerful research tool and a knowledge-sharing community
ProCite - ProCite is a tool for creating citations according to the standard defined by the users.
Weava - Workspace to highlight, organize & collaborate on your research articles.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.