I have no experience in screenwriting software but I thought this looked cool so I'm passing it along.
PlotBot is a new, free online screenwriting site that allows you to collaborate with others on a screenplay. There's a great demo of how the software works right on the front page so it's at least worth a look. While writing you can quickly toggle between dialogue, sluglines, and action. Collaborate with others, track changes, comment on any piece of the script, star your favorite bits (or perhaps bits that need reworking). Enjoy!