This is nowhere near as cool as that Nintendo Wario game ad/video hybrid, or the Honda headlight ad/video that surfaced on Vimeo last year. But it is more useful than both of them combined.
Meet the YouTube piano, a video of piano notes that has on-screen annotations that skip to that particular part of the video and thus the corresponding note. In practice, you could play a song, as some YouTube commenters have done with deep-linked comments. But to be honest, it can't (and won't) sound close to the real thing.
Still, this is about the only useful--and non-annoying use of on-screen YouTube annotations I've seen in a long time. Kudos to its creators, Adam Ben Ezra, Guy Dayan, and Daniel Barak, who run an entire channel of other, similar-functioning instruments. These include a pipe organ, electric guitar, and shaker.
I'm still holding out for a banjo version, so I can strum a little Deliverance.