HTML Tidy was the first (and the dearest to me!) third-party tool that was integrated in HTML-Kit. HTML-Kit may not even exist if it wasn't for the encouragement I got from Dave Raggett who wrote Tidy. HTML-Kit in turn was the first program to add a graphical interface to Tidy.
More recently, Markdown written by John Gruber was integrated in Tools. "File > New > HTML from plain text" menu option is one of the ways you can create Markdown documents. You can of course open *.md files, preview as HTML with a single key/click, and convert Markdown text to HTML as well.
LESS written by Alexis Sellier is another one of these excellent tools that helps improve web coding, especially if you write a lot of CSS. LESS adds support for variables, operations and functions to CSS. Check it out if you haven't already, and you'll soon be able to play with it in HTML-Kit Tools. And yes, support for LESS will go beyond syntax highlighting *.less files!