Google's YouTube website has hidden a playable version of Snake in some videos.
To access the classic arcade game go to a video with the new playback style such as this one of a black screen, click on the video window (it may pause), then press and hold the left arrow key on your keyboard. Keep the key pressed for around three seconds.
We've tried it for ourselves and it works but appears only to be available when using the Flash versions of video not HTML5.
It is not clear when the game was added, but seems only to work on the new YouTube video player introduced in April.