Family Historian by Calico Pie
This genealogy program works directly with GEDCOM files, and is focused on the ability to view and query data. The company web site includes a downloadable demo. The user group web site maintains a wish list. The developer pays attention, and recent free upgrades have implemented many of the requested features.
Family Historian
Family Historian demo
Family Historian Users Group

XSLT Programmer's Reference 2nd Edition by Michael Kay
This book includes a genealogy example. It describes the process of conversion from a GEDCOM file to an XML file, and creation of an XSLT file for conversion to multiple HTML files (very different from the ones here, but a useful starting point). A GEDCOM parser and files for each step are available from the publisher's and author's web sites.
XSLT Programmer's Reference reviews and purchase
Wrox download
Michael Kay GedML download

This XSLT processor was written by Michael Kay. Saxon v 6.5.3 is recommended, and works with the GEDCOM parser.
Saxon v 6.5.3 information
Saxon v 6.5.3 download

This JavaScript and CSS combination expands and contracts HTML lists as if they are trees.
XC information and download

The full version of Saxon requires the Java 2 SDK.
Java 2 SDK v 1.4.2

Genealogy Specs
GEDCOM v 5.5 is currently suported by Family Historian and other genealogy programs. The others are proposals only.
GEDCOM v 5.5 spec
GEDCOM XML v 6.0 spec
GENTECH Genealogical Data Model v 1.1 spec