FlowDocument .xaml and .docx (OpenXML) Writer
This text editor from OpenXML.biz saves to FlowDocument .xaml files or .docx files.
Classes in the System.IO.Packaging namespace give a .Net developer the capability of creating OPC (open packaging convention) documents. .docx is an OPC based file format.
Update: It has a save as that targets .docx, but it appears that "Open" can’t open even the simplest .docx file that word 2007 creates. I guess that is why it is called "Writer"?