While there is a whole lot of junk on YouTube (much like any public forum) there are also quite a few people that post quality content. Here's a few I've found to be worth watching, at least for laymen:
- AsapSCIENCE (https://www.youtube.com/user/AsapSCIENCE)
- Brady Haran's many channels, including:
- Computerphile (https://www.youtube.com/user/Computerphile)
- Numberphile (https://www.youtube.com/user/numberphile)
- Periodic Videos (https://www.youtube.com/user/periodicvideos)
- Sixty Symbols (https://www.youtube.com/user/sixtysymbols)
- Also, several of the professors have their own channels
- CGP Grey (https://www.youtube.com/user/CGPGrey)
- The vlogbrothers (https://www.youtube.com/user/vlogbrothers) and related channels, including:
- CrashCourse (https://www.youtube.com/user/crashcourse)
- SciShow (https://www.youtube.com/user/scishow)
- SciShow Space (https://www.youtube.com/user/scishowspace)
- Head Squeeze (https://www.youtube.com/user/HeadsqueezeTV)
- MinutePhysics (https://www.youtube.com/user/minutephysics) and
- MinuteEarth (https://www.youtube.com/user/minuteearth)
- Some channels from PBS Digital Studios, including:
- It's Okay To Be Smart (https://www.youtube.com/user/itsokaytobesmart)
- PBS Idea Channel (https://www.youtube.com/user/pbsideachannel)
- Some channels from the BBC:
- Earth Unplugged (https://www.youtube.com/user/EarthUnpluggedTV)
- TED-Ed (https://www.youtube.com/user/TEDEducation)
- Veritasium (https://www.youtube.com/user/1veritasium)
- SmarterEveryDay (https://www.youtube.com/user/destinws2)
- Vsauce (https://www.youtube.com/user/Vsauce) and
- Vsauce2 (https://www.youtube.com/user/Vsauce2)
- Khan Academy (https://www.youtube.com/user/khanacademy)
In the more-entertainment-than-education category:
- Vihart (https://www.youtube.com/user/Vihart) - You might not learn much math from her, but you might learn why some of us find math fascinating.
- Scott Manley (https://www.youtube.com/user/szyzyg) - Mostly videos about space games, but there are some occasional videos about orbital mechanics and the like.
- Vsauce3 (https://www.youtube.com/user/Vsauce3) - Mostly game and movie related videos, but sometimes applies science to those games and movies.
And to make this a discussion, what are your thoughts about the accuracy of the science on these channels, and what other YouTube channels get science right?