Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.

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!

Graphity - Graphity is a diagram editor that can be used to quickly and effectively generate drawings and to...

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.

yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.

IOctopus - IOctopus is an online mind map service for a dive deep into the mind for the best ideas.