W3 Online Schools offer top quality free online courses in all things relevant, not least HTML.
W3C's home page for the HTML Activity includes pointers to their specifications for HTML/XHTML and guidelines on how to use HTML/XHTML.
The HTML Writers Guild has provided the very best in online web design training to more than forty-five thousand individuals since 1998. Each online class is taught by experts and working professionals in the field. Their HTML Resource pages contain a wealth of useful material.
HTML Goodies is a site with lots of great tutorials, amongst other things...
UK Web Design Association's site offers a well organized list of resources for HTML authors and Web developers. Many mirror sites make fast access to this information very easy.
Webmonkey: The Web Developer's Resource offers a very useful HTML cheat sheet, special characters, colour codes, browser chart, stylesheets guide, etc. Resources for beginners and more advanced users.
WebCom provides extensive online help for setting up and maintaining successful Web sites.
At DoctorHTML you'll find a list of resources that will be handy for most web site developers.
Pagetutor.com offer award-winning HTML tutorials. You'll find easy to learn material on HTML tables, forms, frames,etc.
The Bells and Whistles site offers a wealth of material for free including hard-to-find HTML and DHTML.