Learn HTML

FireBug

Essential Firefox Addon for web development #1.

Web Developer Toolbar

Essential Firefox Addon for web development #2.

456BereaStreet

One of the best Web Standards blogs out there.

Sitepoint

A forum for asking your CSS and HTML questions, also some good articles.

XHTML: The Definitive Guide

This is a good place to get started, and also something that you will be able to go back to throughout the years as you have questions

bulletproof web design

This book has some cool XHTML and CSS techniques that degrade gracefully

Nikita The Spider

Finds all bad links and other errors in your site, validate all your pages at once, and much more

XHTML Character Entities

By far the best reference for Character Entities

HTML Email Design

Some advice for HTML emails

24 Ways

Similar to this page, 24 Ways is a collection of links to other tutorials.

Digital Web

Great online magazine for learning all kinds of web design techniques.

Vitamin

A online magazine for learning front end development techniques.