In the last couple of days YouTube introduced their newest feature, Video Annotations. As Always there is an instructional video which can be viewed here.
This is a fantastic move by YouTube, which will only increase the traffic of youtube.
The main reason for this, Michelle Lentz pointed out in this article. Is that you can only view the annotations on the YouTube website.
An example of this is when I went to the Viral Video Chart website, as I regularly do, there is a video entitled "Interactive Card Trick" and whilst watching I just thought "how on earth has this gone viral." But once I went to YouTube, I saw it was their new feature.
For now you can only link to YouTube pages, but I'm sure this will change relatively soon, and you will see those websites which are advertised at the end of the video clips, becoming hyperlinks and allowing the viewer to go directly to the website without having to do anythinkg but a click.