Ok - Not really hacks, as much as URLs that utilise youtube in a novel way.
SHOW ME x NON-STOP - This URL allows you to type in a search term and you will be streamed nonstop fullscreen clips. Good for party visuals, stitching up co-workers and wasting your download quota.
Network Awesome - A kind of retro web-channel, lovingly curated from the well stocked bowels of youtube. Presenting 6 hours of thought provoking and eyeopening footage everyday, along with original reviews and companion articles. Network Awesome also has an impressive archive of obscure and esoteric video gems.
Create Your Own Mixtape This site is a kind of advertisment (for some movie called 'Submarine') - however, it's also a very good idea. It lets you compile a mixtape from any youtube clip and share the result. Because, youtube is now the easiest place to find obscure recordings on the internet, obviously...
TWOYOUTUBEVIDEOSANDAMOTHERFUCKINGCROSSFADER.COM - This may be the best website ever made.