The latest release includes the module/container library. This group makes it super easy to add window functions to your site. These are not the popup windows of yesterday. This is about manipulating divs to hide or show on top of your existing content.
Yahoo! Tech was one of the first to implement this library, look at the dropdown menu in the top nav, “what are these words” in the tagnav, interactions for sending/saving products, and much more. It even gives you the ability to insert an iframe under the window if you have z-index issues in IE.