The current editor we use, TinyMCE 3, is being discontinued.
We made the decision to create our own HTML editor to focus on tight integration in Moodle and browsers, as well as the best possible usability and accessibility.
We had a prototype site here but as the new editor is integrated into the upcoming Moodle 2.7 you can now try it out on qa.moodle.net
There are no other prototypes at this time.
Check back later to see more for Moodle 2.8.