Endless Reference is a Youtube channel that focuses on animation reference. Lots of great human locomotion videos from different angles
Alejandro Garcia has a huge Youtube video series all about physics in animation, covering topics like forces, weight shift, jumping and more!
12 Principles of Animation – Alan Becker has made a video series highlighting each of the 12 Principles of Animation as described by Frank Thomas and Ollie Johnston in their book “The Illusion of Life”
Kinovea is video player that allows you to slow down, study, draw and comment. Primarily made for sports enthusiasts but useful for animators too.
Jason Shum is a senior animator at Riot Games and has some awesome free and paid videos on workflow planning, combat, traversal and more!