'Celtx is the world's first all-in-one media pre-production software. It has everything you need to take your story from concept to production. Celtx replaces 'paper, pen & binder' pre-production with a digital approach that's more complete, simpler to work with, and easier to share.'
Doing a screenwriting course a while back I found this software invaluable as most Screenwriting software costs hundreds. It also has the added bonus of allowing you to upload your work to a server and letting you grant access to other parties if you are collaborating with someone.
Couldn't find a free software section so thought I'd put this here.