If you just care about functionality, Leo Editor offers similar features, and adds in being scriptable in Python.

If you also care about usability and zero learning curve, I'm afraid I haven't seen anything similar which is open source, after being searching it for years. The closest thing might be one of several content managers/social networks a la MediaGoblin or Diaspora, or something more flexible you'll need to build your own workflow on some open wiki platform like Twiki or Zwiki.