Slugline
Slugline is a simple, elegant Mac app for the most important part of screenwriting–the writing part.
Slugline automatically formats your writing to precise industry standards. With Slugline, you do everything with text, from changing the format of a paragraph, to adding a new section in your integrated outline.
Slugline uses Fountain, a Markdown-ish format for screenplays, offers autocompletion of locations, characters, &c., and saves your screenplays as plain text, so you can edit them with other Fountain-compatible apps (or any text editor).