Johan The Dog is a free quadruped rig for Maya for use in non-commercial projects.
Advanced Skeleton has been used to rig creatures, props, vehicles and just about anything since 1999. It is free to download but if you want to use it for a commercial project you will have to buy a license.
Grey Matter Gary is a free Maya rig from Squiggly Rigs
Keith Lango has released his collection of tutorials for free! These were originally made between 2005 & 2010 for paid subcribers only but are now available to everyone.
The AZRI Rig is a simple animation rig for Maya 2018 and above, representative of video game projects aimed at retaining a high frame rate when working even on lower-end machines. Featured in the book, ‘Game Anim’
Animbot is an essential free Maya toolset for animators made by Alan Camilo. Highly recommended!
Red Dragon Rig is a free Maya rig made by junior rigger, Elise Mathez
Storyboarder is a free open-source piece of storyboarding software that makes it easy to visualize a story as fast you can draw stick figures.