Freeplane - Freeplane is a powerful and free software for building mind maps.
Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
Linked Ideas - A macOS/iOS apps to treat ideas as links of concepts.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
OmniOutliner - OmniOutliner is a writing app for collecting information, outlining ideas, adding structure to any sort of writing.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.