Henry rig is a free rig made by students from Cogswell University
Built By Colossus Rigs are free and paid Maya rigs available for download. Founded and run by Roman Jeffery.
BOTW Link Rig is a free Maya rig based on the Breath Of The Wild version of Link (similar to the previously posted Zelda Rig). Made by Christoph Schoch.
The Zelda Rig is a free Maya rig made by Christoph Schoch and is available on his Gumroad page
The Link Rig is a free Maya rig made by Christoph Schoch and is available on his Gumroad page
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