The following is right from http://www.inklestudios.com/inklewriter/
inklewriter, helps writers and gamers tell interactive tales with the minimum of fuss. inklewriter keeps your branching story organised, so you can concentrate on what’s important – the writing.
inklewriter is a free tool designed to allow anyone to write and publish interactive stories. It’s perfect for writers who want to try out interactivity, but also for teachers and students looking to mix computer skills and creative writing.
The inklewriter lets you write as you play, branching the story with choices, and then linking those branches back together again. It keeps track of which paths you’ve finished, and which still need to be written.
There’s no set-up, no programming, no drawing diagrams – so there’s nothing between you and the empty page. Oh, and it’s free to use. And once written, you can share your stories with whomever you like.