From Andr Blavier:
TidyGUI is a Windows GUI encapsulation of Dave Raggett's HTML Tidy, a free utility application from the World Wide Web Consortium that helps you clean up your web pages. (You might also be interested by TidyCOM, a COM wrapper for Tidy.)
HTML Tidy is a powerful, efficient, multi-platform application that can be tuned with about 4 dozens options. The major (and probably only) added value of TidyGUI is that it lets you experiment interactively with Tidy's numerous options and save your settings in a configuration file.