KomodoEdit is a free, open source editor based on ActiveStates Komodo IDE and Mozilla/XUL code.

http://www.activestate.com/Products/komodo_ide/edit_features.mhtml

Dita awareness can be added by

• installing the KomodoEdit Dita xpi extension via Tools | Extension manager | Install http://community.activestate.com/komodo-extension/dita
  

or