Multi Window has been in development for some time but until the now it lacked the ability to play YouTube videos. Now with the 2.1 update this is possible.
Why is it cool?
The screenshot shows the trailer of the movie Transcendence on top and the web browser below, with the user reading about relevant topics in Wikipedia.
Another usage scenario could be drawing lessons video in one window and drawing-app in the other window for live practice.
One could think of many usage scenarios.
You can find Multi Window in the store by scanning this QRcode with the Bing Vision lens of your Windows Phone camera.