Etna, a wysiwyg XML RELAX NG- and Gecko-based editor
I took the title for this post from the page for the session description on the XTech 2006 site.
The last modified timestamp on the project page is about a year old, but the current version (v. 0.3.1) may be much more recent. I haven’t yet tried downloading and installing Etna. That’s something I’m looking forward to doing in a VM later on.