Vectr - Free vector graphics editor. A simple yet powerful web and desktop cross-platform tool for everyone.
Freeplane - Freeplane is a powerful and free software for building mind maps.
Vecteezy Editor - Free Online Vector Editor - Design and customize SVG illustrations and graphics quickly and easily in your browser.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.