If you look at the Panels tab of the appearance dialogue window, you will see you can choose what can be hidden or displayed. You will then see that you can add web panels as well. Web panels are bookmarks of web pages that can be access inside the panel window. This is very useful for calendar web pages like scrybe or google calendar and social websites like pownce or twitter. When you bookmark sites that you want to view in the panel window just check the check box labeled "Show in Panel" then you can choose the page from the panel buttons.
Post a comment about other sites you like to use this for.