Given that interactivity is at such a premium, it's not surprising to see streaming video getting an interactive overlay. Mashable writer Amy-Mae Elliott runs down her top 10 interactive YouTube videos.
Note: for those of you expecting video game-level ultraviolent interactivity a la Call of Duty Black Ops (or even the tantrum act-out of Angry Birds), you're sure to be disappointed. However, the constraints of the medium brings out a certain creative flair, with the "choose your own adventure" motif (like that used in the Lego video below) that crosses multiple videos the dominant theme.
Because I'm drawn towards music, my personal pick from this litter is the YouTube radio, which features a set of channels (actually just short lovesongs to YouTube) across multiple genres.
The most impactful and compelling, however, is a "public service parable" that tells the story of a young British man's decision to "Take the knife" (bad idea) or "Don't take the knife" (much better idea). Interactivity with a moral purpose -- it's like the exact cultural opposite of Grand Theft Auto. Nice work.
And if you haven't seen it yet, in the spirit of YouTube-based innovations, here's a video of a set of random musicians from around the globe remixed into a 21st century, asynchronous, anonymous virtual band.