The Animator’s Survival Kit by Richard Williams is an essential book to own whether you’re a beginner or an expert.
Animation Methods is a website hosted by animator David Rodriquez and has links to hundreds of rigs, tools and tutorials.
Frame by Frame provides educational breakdowns of animated and live action clips for animators looking to learn and non-animators who just can’t get enough of the artform
Sam & Amy Rigs are another great set of rigs made by Gabriel Salas
Henry rig is a free rig made by students from Cogswell University
AnimProps is a website where 2D and 3D artists can create an account for free to share, sell or purchase artwork. A big resource for animation rigs.