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 These were originally made between 2005 & 2010 for paid subcribers only but are now available to everyone.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 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.
AnimSelector is a free and super simple Maya script that allows you to quickly setup selection groups that you can later pickup from a GUI or assign to hotkeys.