Live2D has a bunch of software that allows 2D artists to create animations from their artwork. Available in both FREE and PRO versions.
Live2D
Endless Reference
Endless Reference is a Youtube channel that focuses on animation reference. Lots of great human locomotion videos from different angles
Tre Vital Rigs (FREE)
Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!
Bodies in Motion
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 (FREE)
MakeHuman is a free open-source software that allows you to create and pose 3D human characters. Useful for pose reference.
Splatfrog
Splatfrog is a website with hours of lecture/demo videos by veteran animator, Ken Fountain. It has both free and paid videos.
Thinking Animation
Thinking Animation has a great section on the pros and cons of different rigs. As well as a bunch of other resources and tips across the site.
Keyframe MP
Keyframe MP is is a compact, yet powerful, media player designed to meet the needs of animators. Features include scrubbing (with audio), single frame stepping, an adjustable playback range and bookmarks. Free Trial available.
Animator's Resource Kit 