Qiqqa is by far the strongest solution for research and managing pdf's. Zotero, named as a comparison is weak as it does not allow pdf;s to be loaded into its interface - you need to use a pdf reader, which defeats its whole purpose. With Qiqqa documents in a library can be searched together. It has excellent maps of your libraries. It is not prefect, having a few glitches, but still is the only one of its kind.
Google Docs - Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Airborn - Create and edit documents online without sacrificing your privacy
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Bit.ai - Bit is a smart real-time document collaboration platform for teams to create interactive documents, manage digital content and track document insights.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .