Etherpad-lite is an open source collaborative document editor written in Nodejs. I have mine using a MongoDB as the database and a whack load of plugins to improve formatting options. It runs standalone and is very simple to integrate into an LMS like Moodle or a Wordress blog. I have recently updated my instance from v1.5.0 to v1.5.5 and it went seamlessly as always. The greatest achievement has been the work done on accessibility and I swear pads start up a little faster.
Below is my pad I have created that amalgamates the changes from 1.5 to 1.5.5.