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

Duolingo - Duolingo is a free language learning app for iOS, Windows and Android devices. The app makes learning a new language fun by breaking learning into small lessons where you can earn points and move up through the levels. Read more about Duolingo.

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

Eduflow - Digital learning that actually teaches students something

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

Kialo - Kialo is the platform for rational debate. Empowering reason through friendly and open discussions.