It's called Web Clip. You go to the web page in Safari (OS X's browser) and then click the little scissors icon on the toolbar. Then you drag a box around the part of the page you want to see in the dashboard and it creates a live widget from that part of the page. It updates every time you go in to the dashboard.
It does log in for you, since it is actually Safari displaying the page, so as long as you have a cookie saved with your username and password you will stay logged in.
Look
here for a demo of Web Clips.
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moto