Here are listed some tools which might be of use to anyone interested in this project

XML Editors

EditiX is a commercially available XML schema and document editor with a free version which they hide kind of well that can be found here.
Exchanger XML Editor version 3.1 (30 day free eval. available) is Java-based and runs on basically any platform as a result.
<oXygen/> XML Editor is a commercial XML editor with a free trial and great noncommercial pricing ($64 right now).
Sacodeyl Annotator is an open-source tool designed for tagging texts for use in linguistics. However, it is a very intuitive tool for applying XML tags to any sort of text, and so it can be adapted to any sort of use really.
Serna Free XML Editor (Open Source) - available in Windows, Mac, Linux, open source versions in addition to source code
XML Copy Editor is a fast, free, validating editor available for Windows and Linux and in several languages.
XML Spy from Altova is a commercially available, leading XML editor. Altova has a suite of XML tools for database and schema management.

GEDCOM-to-XML Converters
Note, you do not really "need" to install Gramps for using its GEDCOM to XML converter (or XML to Gedcom)
See, command line samples.
GUI and CLI are now independent
One is a web-based direct XSLT Gramps XML viewer, called Gramps Exhibit.
A demo is available (load in memory), and a list of current (1.3.0) XPath(s) is on Gramps' wiki. Minor changes for next version (1.4.0)


TEI Viewer allows easier view of data marked up using TEI.
TextMate an Apple Text Viewer.

Other XML Conversion Tools/Style Sheets

ePub-tools is a set of open-source that converts a variety of XML formats into the e-book standard called ePub. Such tools help envision having a bibliography in a Microsoft Word document that could be imported properly as sources in your genealogy software via BetterGEDCOM.
TEI-OpenOffice Tools provides stylesheets to use TEI P4 and P5 with OpenOffice as well as docx and other formats.
TEI-XSL various style sheets for ePub conversion and several others

Transform Tools

Kernow is an open source tool to make reprocessing transforms easier. It uses Saxon. If you don't know what this means, you almost certainly don't need to.