Celtx is good but a little unpredictable (it’s basically an HTML formatter). I prefer Fade In. It’s cheaper (and easier to use) than Final Draft, has an app for Android or iPhones, and easily connects to Dropbox and other storage spaces for portability.

Fountain, developed by John August and others, is free and very simple; it’s a plain text markup language. (I haven’t used it much, but I think it’s more predictable and reliable than Celtx).