The Bare Bones Guide to HTML
The Bare Bones Guide to HTML lists every official HTML tag in common usage, plus the Netscape extensions.
Weblint
Like picking lint out of a clothes dryer (where did all that fluff come from?), Weblint will scrutinize your web pages to find any potential problems with the HTML you wrote.
How do they do that with HTML?
This site gives several tricks and tips for getting the most mileage out of your web pages.
