Typora - A minimal Markdown reading & writing app.
Freeplane - Freeplane is a powerful and free software for building mind maps.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.