I personally wrote the code behind Sandstorm's "appdemo" feature, which is what the "Try it now" button links to, from the Etherpad front page. Etherpad itself is not written by me; it's written by many other great people.

See e.g. https://github.com/sandstorm-io/sandstorm/commits?author=pau...

I also wrote this blog post about the feature: https://blog.sandstorm.io/news/2015-02-06-app-demo.html

Happy to answer other questions if you have them!