W3.org:

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

XSV: W3C Validator for XML Schema
Web form for checking a schema which is accessible via the Web, and/or schema-validating an instance with a schema of your own.
http://www.w3.org/2001/03/webdata/xsv
Top/Computers/Data_Formats/Markup_Languages/XML/Validation/XML_Schema/Tools

Structured Types in MathML and OpenMath
W3C specifications on Structured Types used in MathML, and how they relate to OpenMath's Small Type System.
http://www.w3.org/TR/mathml-types/#OpenMathSection
Top/Computers/Data_Formats/Markup_Languages/Mathematical

Mathematical Markup Language (MathML?) 1.01 Specification
W3C Recommendation, revision of 7 July 1999.
http://www.w3.org/TR/REC-MathML/
Top/Computers/Data_Formats/Markup_Languages/Mathematical/MathML

Mathematical Markup Language (MathML) 2.0 Working Draft 03
Available as: HTML zip archive, XHTML zip archive, XML zip archive, PDF (screen), PDF (paper).
http://www.w3.org/TR/2000/WD-MathML2-20000211/
Top/Computers/Data_Formats/Markup_Languages/Mathematical/MathML

Editing Mathematics in Amaya
Amaya allows mathematical expressions in web pages, using the Mathematical Markup Language (MathML) specification.
http://www.w3.org/Amaya/User/Math.html
Top/Computers/Data_Formats/Markup_Languages/Mathematical/MathML

EzMath
MathML editor which generates HTML or MathML code. Freeware implementation for Windows 95/NT.
http://www.w3.org/People/Raggett/EzMath/
Top/Computers/Data_Formats/Markup_Languages/Mathematical/MathML

Mathematical Markup Language (MathML)
Official page from the World Wide Web Consortium. Includes the MathML specification, links to related software, and other resources.
http://www.w3.org/Math/
Top/Computers/Data_Formats/Markup_Languages/Mathematical/MathML

W3C Patent Policy
Governs the handling of patents in the process of producing Web standards. The goal of this policy is to assure that Recommendations produced under this policy can be implemented on a Royalty-Free (RF) basis.
http://www.w3.org/Consortium/Patent-Policy
Top/Computers/Data_Formats/Open_Standards

Web Style Sheets
W3C's authoritative overview of style sheets for the web.
http://www.w3.org/Style/
Top/Computers/Data_Formats/Style_Sheets

CSS and XSL: which should I use?
Brief article from W3C with a diagram of the markup-to-presentation process for HTML and XML documents.
http://www.w3.org/Style/CSS-vs-XSL
Top/Computers/Data_Formats/Style_Sheets