Keep in mind that all of these recommendations come with a heavy nerd bias. I like things that are typically on the educational spectrum.
Tested - If you you're as sad as I am that the Mythbusters series is over, you can visit them on Tested.com's YouTube channel. Well, okay, it's mostly Adam, but Jamie is there sometimes. Neither are the bulk of Tested's content, but Adam is a healthy heap of it.
Aquachigger - This fella goes around to places around North America and Europe with a metal detector - usually in a creek or river - and hunts for buried treasure (his own words). Coins, jewelry, sure, but usually he's digging around for Civil War artifacts like cannonballs. It's not all that he does, there's a fistful of general outdoorsy videos where he talks about wildlife, and sometimes he even rescues wild animals. I also happen to be partial to him because he's a WV native. He's also a super happy and positive guy, and his enthusiasm for what he does it enough to keep me glued.
ElectroBOOM - A hefty accent and electrical shocks makes this Canadian immigrant a popular entity on the YouTubes. He's a pretty decent electrical engineer, and you might learn a thing or two while the sadist in you laughs at him shocking himself.
Lindybeige - A Brit who has discussions about nerdy things like obsolete weapons, military strategy, the inaccuracy with which the film industry portrays those things, and sometimes he talks about lindyhop and dancing, and the color beige.
AgentXPQ - A guy named Lev in San Francisco expounds on his day to day experiences while he illustrates/animates them.
Primative Technology - A surprisingly popular channel for a guy who mucks about in the Australian wilderness making waddle and daub huts and clay pots with stone tools. Essentially it's real life Minecraft.
VSauce - An educational channel that usually has something strange and obscure to tickle your mind.
The School of Life - A British channel with a more structured and focused approach than most. It's essentially small crash course lectures on historical figures in various disciplines, their underlying philosophies, and humanities. I particularly enjoy their series on philosophers and their schools of thought.
Wisecrack - A partner channel of School of Life's that has various series including the acquired Thug Notes with Sparky Sweets, Earthling Cinema, and (my favorite) 8-bit Philosophy. Once again it's a pretty good attempt to make headier subjects more digestible.
Team Four Star - Okay, it isn't ALL educational. These guys made it internet famous by condensing and very humourously redubbing the DragonBall Z anime. They do other things, too, but that's their bread and butter, and honestly I think it may be better than the real thing.
The Daily Shed - This is my good friend's channel. He does an 8-10 minute show every weekday on topics varying from the personal, to local and national news, to woodworking, to automotive repair. The show and channel is very much in it's infancy, and while its appeal may not be broad, it's a quality endeavour, and worth a look-see.
Every Frame a Painting - Probably my favorite channel out there right now. An editor from Vancouver, Tony Zhou, takes on aspects of filmmaking in a series wonderful video essays. He even has interesting things to say about Michael Bay. Sold now?