I see a lot of folks here that want to improve their skill level, so I wanted to share a few of my favorite resources and art channels for anyone that's interested, as well as create a designated space for people to post resources that they've found useful. (Maybe this kind of thread existed before; I looked for one but I didn't see it.)
A personal word of advice regarding these links and any material that is designed to help you understand how to improve your art: it only works if you work. You have to do the exercises, follow instructions and stay consistent if you really want to improve.
Drawabox - This is always my first recommendation, because it is the holy bible of design drawing fundamentals.
This site gives you a lesson plan that breaks everything down into manageable tasks that you can take one at a time. You might think the starting bits are too slow for you, and you might want to skip ahead, but resist the urge. Drawabox is all about drilling the fundamentals that you don't even realize you're missing. You won't realize just how little control you have over your own body until you're told to do very simple tasks with high and consistent accuracy. Don't believe me? I dare you to try it. Take it in order from the beginning, and if you complete one of these assignments per week, you WILL improve.
Proko - One of the most consistently good tutorial channels on youtube.
In addition to having possibly the most in-depth breakdown of anatomy drawing that you can get for free on the internet, there's also a ton of tutorials, interviews and exercises spanning a broad range of subjects. There's no intended view order, so if you want to get the most out of these, I'd recommend watching these after you've at least finished all the drills in lesson 2 of Drawabox
BaM Animation - best for folks looking to improve their design philosophy and other fundamentals for good cartooning. These are less tutorials and more active redraws of user-submitted character designs. It's also probably the most entertaining entry on my list.
Sinix - Chill guy that does digital painting tutorials and paintovers. His stuff is not super in-depth, so you probably can't get to his level just by following his advice, but it's a good supplemental channel to follow.
The Joy of Painting with Bob Ross - I am morally and ethically obligated to add Bob Ross to any list about expanding your horizons though art. Paint with the man, let the good vibes flow through you and onto the canvas. Let these meditations bring you closer to enlightenment.
Looking forward to seeing what y'all recommend. Cheers.