I've been working on a writing app for Mac and Windows designed just for comic book writers. It's called Superscript and it's small and simple, but packed with features to make the writing process friendlier:

Automatically formats your script as you type
Automatically numbers panels & pages (even multi-page splashes!)
Spoken word count per dialog bubble, panel and page
Customizable spell check and auto-completion
Include images and comments
And more

Please check it out at https://www.kickstarter.com/projects...va/superscript

If you're interested, please back the project. Reply if you have any questions, feature requests, etc.