Twine is an open-source tool for telling interactive, nonlinear stories. THINK: Choose your own adventure stories.
Twine publishes directly to HTML, so you can post your work nearly anywhere. Anything you create with it is completely free to use any way you like, including for commercial purposes.
Check out 57 North - an interactive "Choose your own adventure" story (for iOS/Android) or Google Search: Twine Stories for many more examples.