I spent all of my adult life in the military which, shockingly, offers little in the way of art education. I've been very fortunate to have been able to gain the ground I have as a cartoonist, but realize how far behind the power curve I am when it comes to understanding basic concepts like color theory, values, and perspective, among others. To tackle that, I've purchased a number of courses and subscribed to some online art instruction sites. I can say in all honesty, from those I've listed below, the experience has been very positive. The knowledge and professionalism of the instructors has been impressive, and I've learned I great deal. Best of all, these are very affordable (ranging from $15-$150 for a course to a monthly fee averaging around $30) when compared to tens or hundreds of thousands of dollars to attend an art school. At any rate, here's the ones I've purchased from or subscribed to in no particular order:
Aaron Blaise's CreatureArtTeacher.com - I discovered Aaron Blaise via YouTube and have been an avid fan ever since. His live drawing videos are absolutely phenomenal and inspiring, which led me to checking out his website. He has a wide variety of course selections covering everything from digital painting in Photoshop, animation, watercolor, to animal anatomy and perspective. I've purchased a number of his courses, all of which have been very helpful. I also purchased some of his Photoshop brush collections and use them regularly for some of my own work.
SVS Learn - Like Aaron Blaise, I discovered SVS Learn by watching Will Terry's YouTube channel about the art and business of being a children's book illustrator. He's immensely talented and his discussions are engaging. The variety of courses available on the SVS site allows for a wide range of possibilities for those of us in learning mode. Even though the primary focus is children's book illustration, there's something there for every working artist - even licensing industry talks. As with Aaron Blaise, you can purchase individual courses or pay a monthly subscription fee which allows access to all available products.
Schoolism - Probably the most impressive, thorough, and extensive online art courses available, at least from my somewhat limited experience. Created by the immensely talented Bobby Chiu, Schoolism has a heavy emphasis on the animated film industry, but the courses are so thorough, and the instructors so knowledgeable, you really can't go wrong with whichever one you choose. I've had a subscription (about $30 a month) and it's worth every penny - a bargain in fact. The degree of detail covered is definitely university-level, and best of all, the instructors provide assignments to check your level of understanding, and critique the artwork you turn in, If you're serious about upping your artistic game, this is the site for you.
Ctrl+Paint - If you're looking to improve your digital painting skills, but your budget has no room left for any extras, this site is for you. Many of these professional-level courses are absolutely free, and those that aren't are still very inexpensive. I've purchased a number of them and have had nothing but positive experiences.
While I'm sure there are many other art instruction sites online, these are the ones I have personally purchased from or subscribed to and can speak about from experience. If there are others that you'd like to recommend, (or advise against) please leave them in the comments below.