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.
Freeplane began as a fork of the Freemind project. While maintaining comparability with Freemind file format, it has far outpaced in features and development. As of 3/5/2022, the latest Freeplane update (1.9.3) is 3/5/2022 vs Freemind was last updated (1.0.1) 2/5/2016. Active, helpful community.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Xmind - Xmind is a brainstorming and mind mapping application.
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
FreeMind - FreeMind is a premier free mind-mapping software written in Java.
JabRef - Graphical Java application for managing bibtex (. bib) databases.JabRef · JabRef Help · JabRef | Blog · OpenOffice/LibreOffice .
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.