Just discovered this gem on YouTube. Just like the Google Pacman interactive logo that was up a while ago, a person on the YouTube staff implemented this neat easter egg where you can play a game of Snake while watching a movie.
To play Snake on YouTube: Pause the movie, then hold the left arrow button.
It also only works on videos that have the new lay-out (horizontal volume slider and sleek timeline).
Pausing/unpausing is also only needed to make sure the flash video has focus. Clicking anything on the player and then holding the key will make it work.
This is especially nice if you have a slower connection and have to wait for the video to load!