Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
StoryMill - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless and many more. Based in MN.
SuperNotecard - Introducing SuperNotecard. SuperNotecard is an online writing tool that features virtual notecards to help arrange facts or scenes, track details, organize paragraphs, and clarify your composition process.
Manuskript - Open-source tool for writers.
Pencil Project - Single-user mockup / wireframing / diagramming tool
Novelize - Novelize is an online novel writing app designed to keep authors on track.