Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
ReadCube - ReadCube is software that manages and organizes research papers and other documents.
Qiqqa - Qiqqa is a free research and reference management software. It can be used in many organizational projects from the academic to the personal to the business endeavor. Read more about Qiqqa.
Citationsy - Citationsy is a reference tool to create citations, reference lists, and bibliographies.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .