Format, Final Draft, etc.
I assume you are really interested in stage plays, since your question seemed to address script format for stage.
For stage plays, you definitely do not need Final Draft or any other dedicated scriptwriting program.
All you need is a good word processing program. Most people have Word anyway. It works just fine. You do, however, have to create a template or download a free one from somewhere. Don't let "template" be a scary word for you. It just refers to a set of paragraph styles that you will be using. I have created all kinds of templates in Word, and in the free programs Open Office.org (OOo) and LibreOffice (LO).
Anyway, use Word, if you have it, or download the free LibreOffice.
(LO is the replacement for Open Office.org. The developers of OOo got tired of dealing with Oracle and have now forked the program over to LibreOffice, which is almost identical to OOo. I just downloaded LO this week after I read something about it here on AW. I recommend LO. Avoid shelling out more money to Microsoft.)
I have always found Celtx just cumbersome to use. I probably have not used it enough to get used to it. I will say, too, that I prefer a completely independent, standalone program like a word processor.
Most of the theaters and contests that I have read about in Writers Market specify that they want a Word .doc file if they ask for an electronic submission. You can save to .doc from LibreOffice. (I recommend the format described as Word 97, Word 2000, and XP; I have used this choice and know it works.)
As for the variety of formats for stage plays, the "production" format in the U.S. is fairly standard with regard to placement of Character Name, Dialogue, and Parentheticals. It is the Action paragraphs that vary. Forget about the appearance of published plays, since they are set up to save paper in printing.