I've been following the Celtx software for some time. I currently use Final Draft for screenwriting, but I have a soft spot for free/open source projects.
Celtx has now released their version 1.0, which usually means it's ready for prime time. I can't say for certain that is true, but they have tweaked a lot of their features and added some new ones. I'm personally interested in the production tools: call sheets, prop lists, storyboard organization, etc. I plan to try out the production features on our upcoming project. I'll post my results.