Qiqqa is recommended for academic researchers, graduate students, and professionals who deal with large volumes of research papers and need an efficient system for organizing and referencing their materials. It is particularly beneficial for those in fields that require extensive literature reviews and citation management.
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.
Annotable - Ultimate image annotation & markup tool
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Good Annotations - A online annotation tool that is perfect for providing feedback to your team.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .
marker.js - Add image annotation feature to your web apps