Krita is a free app for digital painting but also supports 2D animation. It is made by artists that want to see affordable art tools for everyone.
Krita
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.
#Animbreak Challenge
#Animbreak Challenge is a competition created by Jason Shum where you can win cool prizes for animating things, what’s not to like! You can join on Facebook or Polycount
Live2D
Live2D has a bunch of software that allows 2D artists to create animations from their artwork. Available in both FREE and PRO versions.
Keyframe MP
Keyframe MP is is a compact, yet powerful, media player designed to meet the needs of animators. Features include scrubbing (with audio), single frame stepping, an adjustable playback range and bookmarks. Free Trial available.
Drawn to Life
Drawn to Life is a two volume collection of the legendary lectures from long-time Disney animator Walt Stanchfield. For over twenty years, Walt helped breathe life into the new golden age of animation with these teachings at the Walt Disney Animation Studios.
Frame by Frame (Blog)
Frame by Frame is a blog that displays sequential images from various animated films for study and appreciation. It also shows the gif to supply reference for timing.
The Fighters Generation
The Fighters Generation is a world renowned character database… featuring just about every fighting game character in existence. It boasts an unparalleled collection of the best quality artwork, pictures & animations!
Newgrounds
Newgrounds is a popular website for posting animations, games and audio and has been around for over 20 years. Founded by Tom Fulp (also founder of The Behemoth [game developer])
Animator's Resource Kit 