W3.org:

W3.org has 287 URLs listed in 163 Dmoz categories.

W3C: XHTML 1.0 Testimonials
Testimonials for XHTML 1.0.
http://www.w3.org/2000/01/xhtml-test.html
Top/Computers/Data_Formats/Markup_Languages/XHTML/News_and_Media

W3C: XHTML 1.0 Press Release
The World Wide Web Consortium's official press release for XHTML 1.0.
http://www.w3.org/2000/01/xhtml-pressrelease.html.en
Top/Computers/Data_Formats/Markup_Languages/XHTML/News_and_Media

XHTML Modularization Overview
An overview of the modularization of XHTML.
http://www.w3.org/MarkUp/modularization
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference

XHTML 1.0 (Second Edition) Specification
Official W3C specification for the markup language.
http://www.w3.org/TR/xhtml1/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications

Building XHTML Modules
Working draft. Defines how XHTML modules can be built.
http://www.w3.org/TR/xhtml-building/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications

XHTML Basic
A W3C specification for a minimal subset of Modular XHTML.
http://www.w3.org/TR/xhtml-basic/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications

XHTML 1.1 - Module-based XHTML
The current W3C XHTML specification. Includes definitions and documentation for XHTML.
http://www.w3.org/TR/xhtml11/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications

Ruby Annotation
"Ruby" are short runs of text alongside the base text, typically used in East Asian documents to indicate pronunciation or to provide a short annotation. This specification defines markup for ruby, in the form of an XHTML module. (W3C Recommend
http://www.w3.org/TR/ruby/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications

Modularization of XHTML in XML Schema
A methodology for the modularization of XHTML using XML Schema. It allows document authors to modify and extend XHTML in a conformant way.
http://www.w3.org/TR/xhtml-modularization/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications

HLink
Module providing link recognition for the XHTML Family.
http://www.w3.org/TR/hlink/
Top/Computers/Data_Formats/Markup_Languages/XHTML/Reference/Specifications