I always tried to use the space bar to play and pause YouTube videos, but if the focus isn't in the video player itself, it always scrolls the page down. Well no more! I now use the k key to play/pause, and j to jump back 10 seconds. l also jumps forward 10 seconds, but I rarely need that (unless you overshoot jumping back).
There are also others, like shift n and shift p to go to the next or previous video in a playlist. > (shift .) increases playback speed and < (shift ,) decreases it.
Here is a good list, but some of them do require focus to be on the player.