TiddlyMap - An interactive concept- and mind-mapping plugin for the personal note-taking software TiddlyWiki based on the Vis.js library. It's free and open-source!

Freeplane - Freeplane is a powerful and free software for building mind maps.

Minzel - Minzel is a beautiful and free mindmapping tool built for creatives and gives you the power to...

vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.

Wisdomap - Wisdomap Medicine is a study aid that helps medical students create differential diagnoses and suitable management plans from a presenting complaint and structured revision notes.

Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.