The finest app of it’s type: and don’t just take my word for it:

"Screenwriting is an art, and Jeff Schechter conveys both the craft and the soul of the trade. A gifted teacher, he provides methods which will help you both structure and elevate your best ideas.”
David Sacks - Executive Director - The Simpsons

(I have no connection with Mariner other than being a very satisfied customer and occasional beta tester. 30 day fully functional demos available. Mac OS X only sadly.)