This will allow you to navigate a web site in a tree manner. The links of a site will appear as nodes. If you click one its content will be displayed in this window. If you click the little plus on the left, it will expand its own links.
To use as a bookmarklet copy the following link into your toolbar and when you are on a page whose page you want to see here, simply click that bookmark and your mail client will pop up with the subject and message body set to the tinyurl created from the current page.
To use this in html the link for url URL would look like:
To test, you can enter a URL in the form on the lower left or append a url to the following url and put it in the window (I don't know how to escape internal frames):