Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
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.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Pybliographer - Pybliographer is a tool that helps you manage your bibliographic databases in a user-friendly way.
EndNote - Accelerate Your Research. Save time, stay organized, collaborate with colleagues and get published with EndNote 20.
I Librarian - I Librarian is an online tool that comes with a great way to handle and manage your PDF paper and office documents.