Michael Bolin, from Google, created a gadget that loads almost any web page to create tabs that include these sites in iframes. Before adding the gadget, it's recommended to create a new iGoogle tab, but don't forget to uncheck the "I'm feeling lucky" option that populates the tab with gadgets. If you add the gadget to an existing tab, all the current gadgets will be hidden and they'll be visible again after you delete it.
The gadget loads by default Google Calendar's homepage, but you can change the URL from the settings: click on the small arrow from the gadget's title bar and select "Edit settings".
Another useful gadget has a rich text editor and you can use it to create content or to paste code from other pages (like Yahoo Video's embeddable code).
Google, iGoogle, Google Personalized Homepage, Google Calendar, Gadget