ZenPen - A minimalist writing zone, where you can block out all distractions and get to what's important.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.
Telescope - Telescope is an open-source app that lets you easily set up your own online community.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Typefully - Write & publish great tweets, without distractions
Freeplane - Freeplane is a powerful and free software for building mind maps.