Final draft
I work in the film industry and I have to say that both Final Draft and Movie Magic screenwriter are the 2 standards. Although I recomend that when ever you send a script by email send it in .pdf, having a industry standard helps work flow amongst professionals. I.e - Both final draft and movie magic screenwriter are compatible to work with Movie Magic Scheduling.
Whatever program you go for (final draft 6 is what i use, not being very expensive) all I can say is FORGET about Word. If you want to use a non expensive text processor what I do recomend is Pages '06: it comes with an excellent script templet.
Have fun writting stories.