True, 98% of videos are just "look at what my cat can do" or "crazy drivers in Russia" kind of videos. Or video clips made by popular pop/rock artists.
But among them, there are hidden jewels, all offered for free.
Recently, I've been drawn to the inner workings of a cell (a domain of Life Carriers). In this respect, YouTube now allows people all over the world to study for free just by watching online lectures given by university professors. Like this one, given by a Nobel prize winner from Australia, Elizabeth Blackburn, who discovered Telomerase, an enzyme responsible for maintaining telomeres (ends of DNA strands).
Just 20 years ago, in 1994, it would not be possible to have all this content available for free. I surely know what I will be doing when I retire. Watch videos on YouTube...
(Warning: Boring academic stuff full of special terms. Only watch when you have absolutely nothing else to do and after cleaning completely your house/apartment.......
)For those who do not have time to watch. A summary: If you remove stress from your life, if you feel happy and enjoy a healthy, stress-free lifestyle, your telomeres are more likely to get regularly prolonged by telomerase, meaning you will live longer because your DNA will run out of telomeres much later...
I-AM you might also be interested in coursera 
hmm may be a long couple of weeks over here. 