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.
Hotjar - The #1 Leader in Heatmaps, Recordings, Surveys & More. Sign up for a 15-day free trial and start learning from real user behavior today!
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Queue - Queue is a service that helps you promote brands and receive viral growth. The site helps you in a variety of ways, but especially by analyzing market data and giving the best business advice for growing your brand. Read more about Queue.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Instacap - Screen capture, annotation & visual feedback tool for web pages and pdfs
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .