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.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Pybliographer - Pybliographer is a tool that helps you manage your bibliographic databases in a user-friendly way.
Citavi - Citavi is a program for Microsoft Windows that manages literary references and organizes organizational knowledge.
I Librarian - I Librarian is an online tool that comes with a great way to handle and manage your PDF paper and office documents.