Localtube is a youtube videos full bufferer. In other words if your got a crappy internet connection but still wants to see your videos in HD localtube will automatically download locally (in the folder of your choice) the videos of the youtube channels you subscribed to.
Plus it will keep up with the feed of videos, downloading them as soon as new ones are released.
For more informations, please visite the website : http://localtube.org/.