SloMo Ref is a free collection of slow motion animal reference videos on Youtube
Endless Reference is a Youtube channel that focuses on animation reference. Lots of great human locomotion videos from different angles
Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!
Bodies in Motion is an ongoing photographic project that aims to document the power, beauty and possibility of the human body in motion. It is freely accessible to all visitors at preview resolution or you can subscribe for higher quality images.
MakeHuman is a free open-source software that allows you to create and pose 3D human characters. Useful for pose reference.
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 External World Rigs are a free set of rigs for 65 characters created by David O’Reilly
The Hekaton Rig is a free Maya rig available on gumroad from Luis Perez and Tre Vital
The Mery Rig is a Free Rig project by José Manuel García Alvarez and Antonio Méndez Lora and works with Maya 2013+