Red Dragon Rig is a free Maya rig made by junior rigger, Elise Mathez
Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!
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.
The External World Rigs are a free set of rigs for 65 characters created by David O’Reilly
The Hekaton Rig is a free Maya rig available on gumroad from Luis Perez and Tre Vital
The Mery Rig is a Free Rig project by José Manuel García Alvarez and Antonio Méndez Lora and works with Maya 2013+
The Judy Hopps rig is a free Maya rig based on the character from ‘Zootopia’
Highend 3D is a website that has a bunch of free rigs available to download and ranges from Homer Simpson to Spiderman and more!
The Mathilda Rig is a free Maya rig based on Natalie Portman’s character from the movie ‘Leon’