Celtx has appeared a couple of times in this blog. I have started using it for a project where the script-writer is offsite. Google Docs is quite OK, but it is a hell to keep the script in format.
In short, Celtx is an "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."
Celtx is free, has a hybrid desktop-web architecture, with versions for Windows, Mac OS X and Linux. It has presets for different types of media, covering the entire pre-production process (scripts, storyboard, development, breakdown, schedule & reports), allows for collaborative remote team work and also to share your project with the world through Project Central.
Version 2.0 was released some time ago, adding among other things the possibility to use Celtx Studios, a paid project account central providing mass storage, collaboration and e-commerce.