This post is an ongoing one where YouTube channels and other websites that cover music theory and history will be recommended. The channels are going to be added in a rather random order initially. The first website mentioned is a Wiki created by the Society for Music Analysis, the UKs leading learned society for the promotion of the study of music history and theory. The Wiki has links to recommended sources for the study of music theory, history, and skills.
https://societyformusicanalysis.miraheze.org/wiki/Online_resources#Online_resources_listing
Early Music Sources
This channel discusses early music sources (no surprise there) and is probably the best channel for this topic on YouTube at the moment.
https://www.youtube.com/channel/UCJOiqToQ7kiakqTLE7Hdd5g
Olaf the Violin Maker
While this channel is not specifically about music history or theory, it does give some fascinating insights into the the history of the violin from a maker, plus loads of interesting information about the instrument.
https://www.youtube.com/channel/UCe_HpwyNE6agIkPwoaShNlA
PianoPat
Piano music discussed and performed by a French pianist based in Cambridge, England. Discussions of music by composers such as JS Bach, Medtner.
https://www.youtube.com/user/pianopat/featured
Open University (UK) Music Department
An introduction to voice leading and Schenkerian analysis by Dr Robert Samuels from the Open University (UK). There are 8 videos on this topic. This is not a YouTube channel in its own right but is part of the channel OpenLearn from the Open University. The first video is embedded below, but if you go through to YouTube you will see the others.
Check out these great free courses from the Open University (UK)
Each course involves 20 hours of study and is at level 3: Advanced.
Voice-leading analysis of music 1: the foreground
Voice-leading analysis of music 2: the middleground
Voice-leading analysis of music 3: the background
Society for Music Analysis
A small number of videos on analytical topics but there is also a video of an SMA study day featuring YouTubers such as Adam Neely.
https://www.youtube.com/channel/UCNwhklQVgP7GdcggNDk17fg/featured
Classical piano technique and improvisation
Cedarvillemusic is a channel from Cedarville University Department of Music.
https://www.youtube.com/user/cedarvillemusic/featured
Daniel Barenboim
Daniel Barenboim, as they say, needs no introduction.
https://www.youtube.com/channel/UCD7QUSiWc7QBP2XGRMGwjEg
Tudor Music
This channel is about the restoration of partbooks written by Tudor musician John Sadler. The restoration of the partbooks was a project carried out by the University of Oxford.
https://www.youtube.com/channel/UCmfnjmwu8Bytz98zlcxgpOg
Cello playing
emilyplayscello is a channel all about the cello.
https://www.youtube.com/channel/UC95LF0f3HNtVYxnLYAPlwaw
The Spanish (or Classical) guitar
The Spanish guitar Hub with Edina Balczo.
https://www.youtube.com/channel/UCw_6hI_2vETq42wIMnqGW9g
Beethoven’s pianos
This channel is by Mark Beelharing and gives fascinating insights into how Beethoven’s pianos affected his music. There are not many videos on the channel but they are of very high quality.
https://www.youtube.com/channel/UCzgQ3HUWzsKVGCWTLJf_5Cg
Team Recorder
https://www.youtube.com/channel/UCtrtCvRBjJgqZaD17FDg64Q
This is an excellent channel about all things recorder.
Von Huene Workshop, Inc.
https://www.youtube.com/channel/UCpa0DKVefsXAq3TzFU9KLGg
There is only one video on this channel but it is an excellent one giving a detailed account of how a top quality recorder is made.
There will be more channels added soon.
Theory of Music 
Thanks, Barry. Here are my own little YouTube videos: these are terribly old, and I don’t think they look much good nowadays, especially if you don’t have the module they came from. But I quite like the graphs. https://www.youtube.com/playlist?list=PL1E4E365F9865EBA9
Thanks for getting in touch, it is great to hear from you. I will add your videos. I am particularly interested in adding channels that have high quality content but do not get as many views as the most popular channels such as Rick Beato. Everything is going to be a bit random until I get around to organising them into categories later on.
If you’re not being strict about sticking to YouTube, the courses that those videos belong with are here — they’re all free to study:
https://www.open.edu/openlearn/history-the-arts/culture/music/voice-leading-analysis-music-1-the-foreground/content-section-0?active-tab=description-tab
https://www.open.edu/openlearn/history-the-arts/music/voice-leading-analysis-music-2-the-middleground/content-section-0?active-tab=description-tab (dunno why they’ve put a microphone on the front page of that one)
https://www.open.edu/openlearn/history-the-arts/culture/music/voice-leading-analysis-music-3-the-background/content-section-0?active-tab=description-tab (ditto. It’s a nicer microphone, though)