Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Freeplane - Freeplane is a powerful and free software for building mind maps.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
vym - VYM (View Your Mind) is a tool to generate and manipulate maps which show your thoughts.