There are other similar tools out there of course, like... Inform or Quest. But I really like the simple, clean and effective interface of Twine. Too bad I can't draw in it. But it's great for planning story on 'cards' that you can rearrange very easily.
I still remember when I was a kid and I discovered an interactive book for the first time... some colorful yet a bit dark and scary fantasy stuff. On real paper... I was in awe and it stays with me even today. :}
Here's a very small and very weird interactive story... I just regurgitated in no time... for the sake of playing with Twine. Click on the link below to go to...
The Story of Fjord Snulgur.