All you have to do to make a copy of a file on a Mac is hold the "Alt" key ("Control" if you are using a PC) and then click and drag the file. This will make a copy that you can drop anywhere you want by un-clicking.
Pause YouTube with one click, or skip backwards and forwards 10 seconds.
Most people know you can hit the spacebar to pause a YouTube video, but sometimes this causes it to scroll down the page if you haven't already clicked on the video. If you press "K," this will play (or pause) the video every time.
Hitting the "J" key will cause you to go backward 10 seconds, while hitting the "L" key will make you go forward 10 seconds.
If you press "Control" plus "Alt" plus one of the arrow keys, it will rotate your screen that way. Note: this doesn't work for all computers (and only on Windows), but when it does, it can seriously freak someone out.