CeltX is a brilliant application that I used to write my ScriptFrenzy entry, and I strongly suspect it was used by most other ScriptFrenzy winners too.

Far more than just a simple text editor, CeltX is specifically designed for producing scripts for Film, Television, Plays, Radio, Comic Books and more. Since it has built in script formatting, with a single keystroke you can switch between writing action, dialogue, scene headings, character names, shot directions etc.

CeltX creates a 'Project' file for your work, that contains not only your Scripts but character and prop catalogues, shooting schedules, story boards and so on. There's even a commercial service to allow for real-time script collaboration over the Internet. Although the huge number of options can be daunting, there are a few sample project files that help explain things.

CeltX is free and open source (albeit with commercial plug-ins) and is available for Windows, Mac and Linux. It can be downloaded from the official website at http://celtx.com/