This is absolutely brilliant. You can make your own music using a YouTube video!
Hit the play button, wait until the video loads in your browser and then click any of the piano keys inside the YouTube video itself to play some music.
It’s a very creative use of annotations feature inside YouTube. Thanks @Pogue.
Update: You got to click inside the video as shown in the following animation. The annotations will appear only when the video has finished loading.
Also see: Play Video Games on YouTube