Howard Wimshurst has a youtube channel filled with some great animation techniques that apply across both 2D and 3D disciplines.
GDC Tricks of the Trade 2017 is another great talk full of industry veterans giving away free animation tips.
GDC Tricks of the Trade 2016 has some amazing tips from industry veterans. If you haven’t seen this already you need to watch it right now!
Jason Shum is a senior animator at Riot Games and has some awesome free and paid videos on workflow planning, combat, traversal and more!
A workflow breakdown by Adam Turnbull of how he morphed between three different rigs