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Disney's Twelve Basic Principles of Animation is a set of principles of animation introduced by the Disney animators Ollie Johnston and Frank Thomas.
A video playlist explaining the principles can be found here: The 12 Principles of Animation.
This site is full of miscellaneous resources about animation. It's motto is: “Wanna really learn something about animation?”.
The 11 Second Club is a monthly character animation competition. Animators from all over the world participate, animating a character speaking a line of dialogue provided by the club. Throughout the competition, participants can share their progress with each other and critique each other's work. At the end of the month, everyone votes for whose submission they consider the best for that month. The idea is to give animators a chance to practice their skills in a fun, challenging environment.