Joshua Johnson has put a neat post on MacAppStorm about little known shortcuts in OS X, and this one is a particularly great little nugget.
Only today I was watching a mate's video on YouTube, but I wanted to watch a particular portion in slow motion, or frame by frame. Trying to drag the slider so granually in YouTube wasn't working. Then I discovered Josh's neat tip about downloading a YouTube video from Safari.
The basics are start the video in Safari and hit ⌘+Opt-a to bring up the activity window. Then simply find the largest file (usually multiple MB) in the activity window list and double click it. Safari will download it. Typically they come down as .flv files, but if it's got an HD version, try appending &fmt=18 to the URL and you'll get an .mp4 download.