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.
Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.
Pybliographer - Pybliographer is a tool that helps you manage your bibliographic databases in a user-friendly way.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .
I Librarian - I Librarian is an online tool that comes with a great way to handle and manage your PDF paper and office documents.