Line of Action is a really useful site for learning about the human form, anatomy, speed drawing and more. Useful for both 3D and traditional animators!
Line of Action
Wowhead
Wowhead is a site that allows you to view character animations from World of Warcraft. Another great source for game animation.
LOLKing
LOLking is a website that can be used to view League of Legends characters and their animation sets. Great as a source of game animation reference!
Zweifuss
Zweifuss is a website that displays the sprite animations for Street Fighter 3 – Third Strike. Great for reference material and for just admiring the lovely sprite animations.
On Animation
On Animation is a website filled with animation analysis, behind the scenes, podcasts and more. Check it out!
Spungella
Spungella is a blog run by Jean-Denis Haas that is constantly updated with inspiring animations, short films, progression reels and more. A great resource for inspiration!
CG Monks Toolbox
Cg Monks Toolbox is a free toolset for Maya that includes a large selection of handy animation and rigging tools
Animation Exchange (2017)
Animation Exchange was an 8 hour twitch stream hosted by a bunch of lovely animation veterans. The aim was to take the knowledge sharing and powerful community of art and development conferences and make it accessible to all, and it was awesome!
Alejandro Garcia
Alejandro Garcia has a huge Youtube video series all about physics in animation, covering topics like forces, weight shift, jumping and more!
Animator's Resource Kit 