It has been a long time since I last tried to write a play, and I hated it, not the play (which became a short story), but writing one. The word processors made working with script layout a pain, and I detested marking which character was speaking. I still prefer prose and poetry, but I was wondering are there any tools that makes writing a script easier? Are any of them open source?
Have you tried Scrivener? I haven't used it for screenwriting, but it looks like it's got a great tool for it. It's not open source, but $45 is fairly cheap.
Celtx is free, mostly open-source (it has a nonstandard license and there are some closed components) and very good. It does script formatting, plus a bunch of other organisational stuff (keeping track of characters, storyboards). I haven't used it for a while, but I found it very intuitive and so on. There is even some collaborative stuff built into it, though you have to pay to get access to that (or at least you did when I was using it -- these are the closed-source components).