They done some nice work to it but now they've added versioning in their Celtx Studios.
What the buggery does that mean?
It's like this: Let's say you write a draft of your script. Then you write the second draft. The first draft still exists on the Celtx system.
Now you think - what if I completely get rid of that character? Well, you can. The previous versions exist PLUS you can back to an earlier version while still keeping the one with the removed character.
You can compare them to see where the actual differences lie, cut and paste between create as many different "branches" of your script as you want and so on.
But remember Celtx isn't just a screenwriting system (it's also an AV writing system, Comic writing ...) but more than that. It's a complete movie production control system. And every element of it has the same version control.
It does all sort of clever stuff that you need when you produce a film of any length - down to scheduling, prop lists and call sheets, and other stuff you need that I don't know about. All integrated so that your work is kept to a minimum. It also does collaboration so you can have different people working on different aspects of the whole project. Or you can have multiple authors on the same script.
What else? Oh yes, Celtx Studios allows you to create web "views" of your project so that you can give other people access to see parts of your project (like the script perhaps). But still maintain security on your whole project.
The downloadable Celtx software is still free. The new versioning (et al) option, through Celtx Studios, costs $50 per year. I'll pay that just for the versioning, and these guys have been doing all this work for free for a very long time, I don't begrudge them a subscription.
I don't get a commission.
What's on the turntable? "Song for Sharon" by Joni Mitchell from "Hejira"