It's ridiculous to post above 0.9.4 announcement, but anyway.
Here's a little addon that allows you to create a menu with popups and items etc.
It isn't a full blown GUI. Whenever a menu item is selected it returns the name of the item and it's state. You can do what you want with that. If you are familiar with the Windows API you can go further with resources and make modeless dialogs that control application variables directly.
I will be interested if people find it useful. I am using it with success.