Final Draft
Price: $199.99
Won't put dialogue on the pages of your screenplay for you, but it'll do almost everything else.

Hollywood Screenwriter
Price: $49.99
Designed for writing spec scripts designed to get the attention of stage or screen producers.

Movie Magic Screenwriter
Price: $189.99
Automatically formats your script & does a good job of guessing what you want.

RoughDraft
Price: Free
Specifically designed for writers of novels, short stories, articles, poetry, plays and screenplays.

Dramatica Pro
Price:$199.99
Focus more on your stories and less on the mechanics of storytelling

Scriptwriters Toolkit
Price: Free
MS Word templates for writing Screenplays, Stageplays and Radio Plays.

SquareNote
Price: Free - $35.00
Like "index cards" on a PC - with the power and speed of the computer.

StoryBuilder™
Price: $129.95
CAD tool that helps you rapidly create outlines that can be turned into effective, saleable stories.

StoryCraft
Price: $79.00
The classic plot-development and story-structuring tool.

Writer's Dreamkit
Price: $49.99
Writing package that provides guidance but isn't overbearing.