How long this has been available, who knows, but someone just found an Easter egg in YouTube that allows you to play a game of Snake on the video while it's paused or plays.
I've confirmed this for myself - it takes some doing, and maybe it's because I'm using a Mac, but if you press the up and left arrows simultaneously Update: Just hold down the left arrow; up is not necessary, it triggers the game of Snake. Snake, of course, is a game that's been around forever - move your snake around the screen and gobble up pellets, increasing your length as you do, without hitting the edges.
The Easter egg also seems to be tied to a video being categorised "gaming." I'm not sure if you need to be on the YouTube page itself or if it works with an embedded video. I tried it in a few other non-gaming videos and couldn't make it work. All that said, the command was very unreliable for me even in the gaming videos, so I can't be sure.
Final update: To the best of our knowledge: Just hit or hold down the left arrow; you have to be on the actual video page, not embedded or on the channel page; some say the clip has to be fully loaded, others say they can do it while it is loading.
Big thanks to Kotaku reader Save Me for the tip!