I've decided to put together a small list of tutorials I found interesting/useful/well-written/with awesome results. I recommend them all, no matter if you just got started or if you're really good; they are definitely worth a look, and maybe you'll learn something new.
I sorted them by what they focus on. I may make another similar journal with more areas later.
Colour pencils (or a mix of other traditional media):
Watercolour (and coffee):
Digital art (cartoony/anime):
Digital art (painting, semirealism, realism):