We've always known you can press spacebar to play and pause a YouTube video, or jump to certain points with the number keys. You can also use J, K, and L, as Back, Pause and Forward buttons.
Pressing K does the same thing as spacebar: it pauses or plays the video. However, the two keys on either side of it, J and L, act as Previous and Next buttons, respectively, just like a regular remote control. Pressing J will take you 12 seconds back in the video, and pressing L will take you 12 seconds forward. Just another handy little shortcut for kicking back with YouTube videos, no mouse necessary.
Image: Luke Jones.