I Want to be an Animator is a Youtube collection of free Lessons, Tutorials and Tips about 3D Animation, mostly focusing on the basics and principles.
Darkstalkers – 12 Principles is an article that explains the 12 principles of animation through the pixel animation of Darkstalkers
Jonathan Abenhaim has an amazing YouTube channel that covers a load of tips, tricks and tutorials on animation. He posts regularly so make sure to subscribe!
Craft is a site that gathers high-quality material from films, graphic novels and other visual media and lets you explore how other professionals crafted it. Giving you access to source files from animators and artists. Free Trial is available.
Splatfrog is a website with hours of lecture/demo videos by veteran animator, Ken Fountain. It has both free and paid videos.
Thinking Animation has a great section on the pros and cons of different rigs. As well as a bunch of other resources and tips across the site.
Kenny Roy is a 16 year veteran of the animation industry, and 7 year Mentor from Animation Mentor. Sign up to his members section for Monthly Videos, In-Depth Articles and more!