W3C - World Wide Web Consortium

You've probably at least heard of this organization, but may not know the scope of their responsibility, therefore I thought I'd briefly touch on it, then point out some extremely useful links for you web developer types.

The W3C is the group responsible for defining guidelines used on the Internet, including HTML, CSS, XML, etc. The recognized inventor of the Web, Tim Berners-Lee originally founded the organization in 1994 while working at MIT. Once they planted their roots, they began to work on many different specifications for how content should be developed in order to be as compatible as possible with all of the various resources tied to the Internet. If you want to know the "book answer" on a web standard, this is the place to go.

Aside from just defining the specifications, they've developed some extremely useful tools for developers to use in order to be sure their code is as compatible as possible. Here are just a few of the many valuable links you'll find on their site:

Introduction to HTML 4
XML In 10 Points
HTML Validator
CSS Validator
Open Source Software