The Wilderness Downtown
I'm pretty jazzed about this Arcade Fire music video, The Wilderness Downtown (can you even call it a music video as it's not in a traditional format?). It uses HTML5 to synch the audio timecode to corresponding film clips in differently sized windows, integrate Google Map's API for creating a 3D environment, and creates a canvas (<canvas>) for the viewer to interact with. A pretty stellar presentation.
It's really interesting to see if more artists will build upon this and other digital technology to expand the meaning of a music video and how listeners interact with it.
Also to note is Bjork's apps for her last album, Biophilia.