Character entity references in HTML 4 which will validate with the W3C. Dreamweaver does not use these entities automatically.
The W3C has long offered HTML Tidy, a wonderful tool that changes your HTML to XHTML with no pain, but it was a Windows only application. Now we have Tidy for Mac OS and Mac users can convert to XHTML painlessly, too.
HTML 4.01 Reference is a page within the w3schools site that lists all the HTML tags and shows which ones have been deprecated. It also links to demos and useful information about each tag listed. Excellent resource page.
How to create accessible forms from WebAIM is aimed at people who want to make a form usable by a screen reader, but also looks at good form skills in general.
htmlcompendium.org – The Compendium of HTML Elements is an in-depth resource with information on tags, maths, css, servers and more. At this point it is still free, but like many valuable sites hurt by the loss of advertising revenue on the internet recently there is talk of a subscription fee.
The HTML Writers Guild recently merged with the International Webmaster Association. Online classes and other resources.