I just read this article on free screenwriting software: http://www.videomaker.com/downloads/free-screenwriting-software. An interesting look at a few of the options available.
I must recommend against Trelby. The app can’t do basic text-formatting such as underlining or italics, and the developer gets very prickly at people who ask this. It also doesn’t embed fonts properly into PDFs, so someone who receives only the PDF might not be able to print the script.
Not mentioned in the article is CinemaVision Fountain Editor. It formats a script, and does underline and italicize. Beware: it does not format the title page correctly!
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