I've been actively involved in a few different art critique communities for the past 4 years, and I've noticed that most people seem to struggle with a lot of the same things. Here are the resources I suggest to people the most. Feel free to add onto this!
This composition guide by genicecream here on Tumblr, which was the first thing to make composition really click for me:
This absolutely excellent guide by SpongeBob storyboard director Sherm Cohen, which goes into a lot of different things, but mainly storytelling and posing. It is critical for anyone working with storytelling in art, even if it's not as stylized as SpongeBob!:
This guide to edges by Marco Bucci on YouTube:
This video series about design theory by Sinix that is absolutely essential for character design, composition, and pretty much doing any work with visual art at all.
The Schweizer guide to spotting tangents explains itself. Everyone should read this. There is also an updated version here with even more information!
Pick a head angle using a 3D model and be given a matching human or animal reference photo (credit to @selfish-ghost for initially posting these to Tumblr).
Line of Action is a well-known reference photo website that provides randomized references set to a timer. They currently offer full figures, animals (including bugs), hands and feet, faces, basic shapes, and environment scenes. They have a really awesome tutorial for figure drawing here that only takes 15 minutes!
Adorkastock has both a huge backlog of reference photos with a TON of different action poses (of models with varying body types) and another timed reference photo tool!
Quickposes and Sketchdaily also offer timed reference photo tools.
Earthsworld is a wonderful collection of high quality candid reference photos of people spanning all the way back to 2003. This is an excellent resource if you're looking to break out of sameface syndrome.
This virtual lighting studio by zvork allows you to make custom lighting setups on 4 different 3D models (including 1 darker skinned model).
William Nguyen on Artstation has male and female Asaro-inspired 3D head models that are rotatable with customizable lighting, intended to show the planes of the face.
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