The short answer: whatever you’d like! The HTML widget is there to do all the things that the other widgets don’t.
Maybe you coded your own widget and want to put it on your Guild Launch site… or maybe you just want to put a picture of a kitten in a funny hat on the main page where everyone can see it. Or embed a video. Set up a donation link to a charity. Moving your primary navigation to the right sidebar. All that and more is possible with the HTML widget.
The baseline is the WYSIWYG (What You See Is What You Get) view. Think of this as a very simple, Microsoft Word. You can type text, highlight it and click the B button to make the text bold, for example. This is the easiest way to use the HTML widget.
If you type something and highlight it, you will be able to turn it into a link by clicking the 3 chain link icon. Using this view, you could easily type a list of Guild Officers and link each name to their Member Profile or add a flash video by clicking the movie strip icon and providing it with a URL.
What if you know HTML or want to paste HTML into the widget to display something you've coded? Easy! Switch to HTML view by clicking the HTML button. In the pop-up you can insert HTML to display that item on your site. You would also paste your code for an HTML based Chat room in the Code View box.
Remember that the HTML Widget's flexibility is its power. The Linkbox Widget is great to display your links, but what if you want to use images instead of text? What if you want the text to be colored fireball fuchsia or displayed in a certain order? The HTML Widget solves these problems and is very easy to use.