I remember when Harlem Shake was popular. Everyone shared that on Facebook. Best one yet.
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If you go to Google.com and press 'I'm feeling lucky' without typing anything, you'll see every google logo ever used.
Not really a YouTube development but this is very handy, if you take a youtube video url, but instead of youtube.com you put ss before it (ssyoutube.com/vidurlhere) it takes you to a site where you can download the video.
Just found this out, very handy.