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.
I Want to be an Animator
Rise and Spline
Rise and Spline is a weekly masterclass hosted by David Gibson and featuring a bunch of amazing animators. Tune in on Twitch every Saturday morning!
Keith Lango: Checklist
Keith Lango has a great checklist full of tips on how to polish and finalize an animation. You can also download it as a PDF from HERE
Darkstalkers – 12 Principles
Darkstalkers – 12 Principles is an article that explains the 12 principles of animation through the pixel animation of Darkstalkers
Jonathan Abenhaim
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
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
Splatfrog is a website with hours of lecture/demo videos by veteran animator, Ken Fountain. It has both free and paid videos.
Thinking Animation
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.
Animator's Resource Kit 