For a long time, there has been a live preview site for reStructuredText: http://rst.ninjs.org/. It is really fantastic for learning the language. The immediate feedback is really valuable in helping you expirment and see how things work.
I think that this tool would be great for people to have with Sphinx as well. I know a lot of people use Sphinx, and then end up on that page, and random things don’t work because they are extensions to reStructuredText. So, I went ahead and forked the app to support Sphinx.
I present: http://livesphinx.herokuapp.com/.