I use Final Draft....
but here's a couple of screenwriting programs that are free:
http://www.scripped.com/
http://celtx.com/

Shea, I just found scripped.com a few weeks back. Interesting to see that Ed Burns is involved with that.

It was also interesting to find out Ed is one of the producers of the series "Generation Kill" It's a great show, one of the very few on TV I'll watch.
I went to an interview of his at a Mac Store on 3rd St., Santa Monica
He stayed longer then he needed, and was very giving with his time. He's one of the reasons I started writing.
Great Commentary on his movie "The Brothers McMullen" especially for independent filmmakers.

I go here every day

http://hollywoodwiretap.com/

http://www.cinematical.com/