Now that the Google Sites HTML box has been out for a while for those without Google Apps, I've had some time to fool around with it.
I couldn't embed the Google + gadget (strange errors I couldn't fix kept flagging my code to my unpublishable). I couldn't use my usual slideshow code, which uses jQuery. So I started assuming that nothing would work with the HTML that I really wanted.
Free Google Sites Slideshow Code
The only thing I've effectively used it for is the sample slideshow from: http://support.google.com/sites/bin/answer.py?hl=en&answer=2500646 . I nicely customized this slideshow and placed it on this page with its code for all to use. Just copy + paste and insert the image URLs.
|Click here to go to the HTML box testing page. With a customize slideshow.|
I'm going to stick with making custom gadgets, because even though it's more arduous, I can make the plugins I want to make. I did notice that the HTML box isn't an iframe like gadgets, so I think that creates room for interesting use of embedding code on your pages. I've had quite an interest in Google Analytics with Google Sites for event tracking.