This is YouTube's first "interactive" video.
With its new annotation feature, the site is allowing makers to tag their clips with running commentary, a la VH1's Pop Up Video. You can add text boxes to explain what's happening, put in 'spotlights' to point the viewer's eye to one place on the screen and perhaps most interestingly, you can add links. Linking inside the clip will allow people to connect strings of videos to one another.
The obvious application is as a kind of choose your own adventure, in which you can make decisions about where to go next (see the card trick video). My guess is people will find plenty of inventive uses for in-video linking -- it's a nice way to direct viewers to other material, or to the video which your video is referring.
Here's my little attempt at using annotations -- took me a little more than an hour...
The other Google Visualizer post is here.
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