Emacs

From flickr user tsmall, Attribution-ShareAlike 2.0 Generic license

Duck, Duck, Gnu: Mallard and DocBook 5 support in Emacs

Edit: I recommend checking out Paul Frields’ post about Emacs and nXML mode, and also looking at the Fedora wiki page that he created.  I think his approach is a bit simpler and cleaner than what I’ve provided below.  Feel free to peruse what I’ve written, though, as some of my links to “getting started” [...]