Final Draft - Use your creative energy to focus on the content; let Final Draft take care of the style.
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.
Celtx - Celtx is a scriptwriting software platform with applications in a wide range of mediums but that specializes in helping screenwriters.
Novelize - Novelize is an online novel writing app designed to keep authors on track.
Scrivener - Scrivener is a content-generation tool for composing and structuring documents.
Manuskript - Open-source tool for writers.