www.pachyderm.org I requested a demo account and rcvd a quick response. So I was up early this morning looking at the documentation and noodling around the authoring interface. I am extremely impressed! Not only is the editing environment well designed and easy to use, but the documentation is too. Everything worked flawlessly and the published output is quite astonishing. I say that because of the examples on the site. Check out Technologies for Learning.
I know there are other open source initiatives out there, but the design effort put into Pachyderm is beyond anything I have seen to date.