Polymath, entrepreneur, and fast talker Hank Green offers up a really very useful set of YouTube tricks and techniques.

Here are the keys he mentions:
j - Jump back 10 seconds
k - Pause and un-pause playback
l - Leap forward 10 seconds
, (comma) - Move paused video back one frame
. (period) - Move paused video ahead one frame
< (shift-comma) - Slow down playback > (shift-period - Speed up playback
M - Mute and un-mute the sound
Up arrow - Louder
Down arrow - Quieter
Left arrow - Jump back 5 seconds
Right arrow - Leap forward 5 seconds
0 - Go to the start of the video
1 - Go to a point 1/10 of the way through the video
2 - Go to a point 2/10 of the way through the video
...
9 - Go to a point 9/10 of the way through the video

And from comments on the video:
/ - Jump to the search box (works a lot of places in Google products)
O (shift-o) - Change the color of closed captions text
W (shift-w) - Change the color of closed captions background
(Turn on captions by mousing over the video and clicking the CC button.)

Finally, right-click on the video (or Control-click on a Mac) and you'll get a menu of options that includes "Loop" and "Copy embed code" (which copies some HTML to your computer's clipboard than can then be inserted into a web page to cause the video to appear there).

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