The Neurocentric recommended that I link this free pre-production software. It won't write a play for you, but it will help you to organise your play. As the website says: You can use Celtx for the entire pre-production process - write scripts, storyboard scenes and sequences, develop characters, breakdown & tag elements, schedule production, and prepare detailed and informative production reports for cast and crew.

You can find out more about it and download it here.