Cocktail Flow is a very popular and highly praised Windows Phone application.
In this video, Balint Orosz, one of the developers of Cocktail Flow does a very quick demo of the app and talks about how they created, how long it took, and what is coming next for them. This video is aimed at developers so it does not do the app justice with regards to the demo. This video was created as part of the "An update from the Windows Phone developers" blog post. You might want to read the post to make most sense of the video.
Better video demonstrating Cocktail Flow. (pruty, ain't it?)