Scripped Writer is a free web-based script writing software application you can use right in your web browser. Unlike other screenwriting software, Scripped Writer is built specifically with the needs of the writer in mind. That means it has all the functionality you need without the clutter of features you'll never use.
No installation is required. With a browser based web application you can use Scripped Writer without downloading software and installing it on your machine.
(If you are looking for free scriptwriting software, you should also consider Celtx.)