Penrose - Create beautiful diagrams just by typing notation in plain text.
Visual Understanding Environment - Visual Understanding Environment (VUE) is a mind-mapping and concept-mapping tool.
draw.io - Online diagramming application
TheBrain - TheBrain: The Ultimate Digital Memory
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.
Inspiration - Inspiration K-12 supports visual thinking techniques, enabling students to easily create and update...