HTML Reference Guides

HTML Characters Chart  (webmonkey) (about.com) (answers.com)

HTML Cheat Sheet

HTML Color Chart
Another HTML Color Chart
Another HTML Color Chart

Font Guide

Fonts / Colors / XHTML / CSS / JS / DOM / Regular Expressions

Reference Links

Dynamic Drive (DHTML/JS)
HTML Goodies (DHTML)
The Javascript Source (JS)
HTML5/CSS3 Browser Compatibility Chart

Tutorials (HTML/JavaScript/CSS/Flash)

Echo Echo
W3Schools (for everything)
Javascript Kit
HTML 4.01 Tutorial

Cool Javascript

Location object
Javascript Kit

Validators

Valid XHTML 1.0! Valid CSS! Mobile validator [Valid RSS] HTML / XHTML validator CSS validator Link Checker (for dead links) Validated by HTML Validator

XML Schema Validator   -   Find Broken Links, Redirects & Site Crawl Tool