I was looking for info before buying an upgrade to the latest version of Final Draft (decided against it, no advantages over the version I've got) and came across a link to this free writing software recommended by many.
Haven't tested it, so don't know if it's as good as they say. One thing for certain is that it's not industry standard and scripts would have to be converted to PDFs before being sent out (as opposed to Final Draft files, which just about every production company has the software for). But having managed without dedicated screenwriting software for a dozen years before switching, I can testify that a program taking care of layout while you focus on the writing is a Good Thing.
If you've tried Celtx and have an opinion on it, feel free to add a comment.