CSE HTML Validator Professional
To be clear, this isn't a WYSIWYG tool or intended for use by people who do not know HTML. It assumes you know what you're doing and just need to make sure you've dotted all your "I"s--or more technically, closed all your markups. It checks for compliance with current standards, looking for possible as well as actual errors--it alerts you to tags with no content, for example, or that a space should precede a backslash for backwards compatibility.
In addition to validation, CSE HTML Validator Pro is a tool for creating and editing your own documents. It includes a small number of basic templates (framed, not framed, XHTML, and so on), and has good syntax highlighting and 'code completion' features. It will, of course, format your HTML for you on command. Despite the name "HTML Validator", it will open PERL and basically anything else stored as pure text, however, it does not possess any features to edit them--when I asked it to validate a PERL file, it treated it as HTML. This feature is useful, I suppose, for people who are editing multiple files and like to have them all in one place-- CSE HTML Validator Pro uses a tabbed interface.
Validation results are displayed in their own set of tabs--view them all, or grouped by type. You can change many of the parameters used by the validation engine, as well.
CSE HTML Validator Professional is a powerful tool for anyone who does a lot of HTML editing or checking. It is especially useful for anyone who must post HTML files written by others--this program performs a nice "sanity check" before potentially broken files make their way to your corporate Web site.
Note: The trial lasts for 30 days or one hundred validations, whichever comes first.