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.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Raindrop.io - All your articles, photos, video & content from web & apps in one place.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Tagpacker - A free tool to quickly collect, organize, and share your favorite links.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .
Diigo - Diigo is a powerful research tool and a knowledge-sharing community