In the case of the site he appears to be using a plugin, but Xtreme has similar capabilities.
With the base Xtreme, you have multi column widget area that could be used, you also have Classes you can use on your own HTML to mix 3 columns and 1 column as is on the page. there is a tutorial below
http://forum.weavertheme.com/discussion/11220/weaver-xtreme-tips-and-tutorial-part-5
If you have “Plus” then the full functionality of Custom Widgets area with any kind of layouts is built in as you can see on the demo site and as explained in the tutorial below.
http://demo.weavertheme.com/insertion-areas/
http://forum.weavertheme.com/discussion/10342/weaver-xtreme-tips-and-tutorials-part-1
The Custom Widget area in Plus are actually more powerful than the plugin he is using. As you shrink the browser on the site you referenced, you will see the boxes do not keep the same height. Xtreme with “Plus” has an option to dynamically equalize widget height in rows of widgets.
So the layout your referenced is easily created in Xtreme, and we can help with it when the times come