Reji - Improve your vocabulary hassle-free 📚
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.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Lexios - Learn vocabulary effortlessy while browsing the web
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.