I saw a useful video on the YouTube Trending page today. Here are a few of the YouTube tips featured in it.

shift + < = decrease playback speed
shift + > = increase playback speed

J = go back 10 seconds
K = pause
L = go forward 10 seconds

When paused:
, = go back one frame
. = go forward one frame

0 = return to start of video
9 = jump to the 90% point
1-8 for anywhere in between

up = increase volume
down = decrease volume
left = back 5 seconds
right = back 5 seconds