Secondly, with a few exceptions most of these channels do not focus specifically upon the fantasy or speculative genres, and they're not even channels by writers. The majority of YouTube channels below are, I believe, informative, often entertaining and well worth the time of any fantasy writer. Also, this list is not meant to be exhaustive, but is merely made of channels I personally have found useful and interesting. The videos shown are only a sampling, and should not be considered representative of everything available at the individual channel.
Scholagladiatoria
The host here is Matt Easton, who has more than a dozen years experience teaching Historical European Martial Arts (HEMA). Of all the YouTube channels pertaining to European sword fighting, I personally feel this one is the most knowledgeable. Even if you think you know your stuff when it comes to swords, I bet you will find some surprises here and learn a few things. Also, swords aren't the only subject matter here, as Matt gets into other period weapons.Skallagrim
I might be a bit biased when it comes to Skall's channel because I mailed him (and his girlfriend, Cara) a package a while back and it was featured on the channel. Still, I think this is one of the best sites for learning about European historical swordsmanship, other historical weapons, armor, etc. Though not as experienced as someone like Matt Easton (Skall often calls himself a beginner), there is still much to pick up here. For one thing, most viewers also are likely to be beginners, or only armchair swordsmen (waving my hand here), and it can help them to study the basics along with another beginner. But swords and training aren't all that appear on this channel. If you are interested, Skall sometimes has a firearms video or a more philosophical video or even a fun, entertaining video featuring a bit of role playing.Lindybeige
Of all the channels mentioned here, this one is the most entertaining and the most funny, at least in my opinion. Lindybeige himself has some experience concerning European historical arms, and most of his videos make specific points about weaponry and the like while bringing out some humor. Occasionally he will include a travel video or simply an entertaining video, and sometimes he will dissect historical or fantasy movies or movie scenes, all with some chuckles.ThegnThrand
At first glance, this channel looks like it's run by a couple of good-ole-boys who like to stab things in their back yard. Then you find out they're in Texas, so they really are a couple of good-ole-boys who like to stab things in their back yard. And when I say "things," I don't mean they're serial killers knocking off their latest victims for the world to see, but that they are testing historical weapons against the likes of cardboard boxes, wooden targets, dummies, etc. One of the nice things about this channel is you get to see historical weapons, mostly sharp, pointy things, used in a practical format. You get to see the weapons used properly, and you get to watch of what the weapons are capable. Also, the two guys behind this channel, Thrand and Eldgrimr, do a good job of debunking some of the nonsense you might find on "historical" television and movies and the like. And they are more than willing to work through any debates or conversations concerning the different uses of arms historically, all from a practical viewpoint. Fans of historical Vikings should be more than pleased with this channel.Honorable mentions
Nerdarchy: This channel focuses upon tabletop role playing games, usually Dungeons and Dragons, but quite often the information discussed can be of interest to fantasy fiction writers. The regular crew here are Ryan, Ted, Dave and Nate, but friends of theirs show up from time to time, as well. There are videos here of actual gameplay, so you can learn about tabletop RPGs if you're not in the know, but this channel tends to focus upon group conversations about aspects of the game, many of which can also be related to writing. Storytelling, character arcs, world building and more are common topics of conversation. As the gang often says, "for nerds, by nerds."
hickok45: Perhaps a little less useful to the fantasy writer, this channel focuses upon firearms of all sorts from history up to modern day. The emphasis is upon modern and semi-modern firearms, but occasionally there is a video about black powder weapons and the like. Still, to some extent or other the information here should be of interest and worth study for most fiction writers. Besides, the host, hickok45 (and sometimes his son, John), is pretty entertaining and offers plenty of hands-on knowledge and experience.