Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!
Tre Vital Rigs (FREE)
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.
Drawn to Life
Drawn to Life is a two volume collection of the legendary lectures from long-time Disney animator Walt Stanchfield. For over twenty years, Walt helped breathe life into the new golden age of animation with these teachings at the Walt Disney Animation Studios.
Frame by Frame (Blog)
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.
External World Rigs (FREE)
The External World Rigs are a free set of rigs for 65 characters created by David O’Reilly
Hekaton Rig (FREE)
The Hekaton Rig is a free Maya rig available on gumroad from Luis Perez and Tre Vital
Animator's Resource Kit 