A Sourceforge project called Vex seems to have crude support for DITA: http://vex.sourceforge.net
The release notes dont mention DITA, but if you install Vex (which is just an implementation of Eclipse), you'll notice their sample docs have an entire DITA set. You can import the Derby DITA files into your project, but the projects dont recognize the DITA files as DITA unless you change the extensions from .dita to .xml. And it doesnt recognize ditamaps at all. The visual representation leaves a lot to be desired. Someone may want to play with this to maybe improve it to a more useful product, but for now, its a start...
