The Link Rig is a free Maya rig made by Christoph Schoch and is available on his Gumroad page
MocapX is a Facial motion capture app for the iPhone that streams the mocap data to Maya in real-time
Grey Matter Gary is a free Maya rig from Squiggly Rigs
The Setup Machine enables Maya users to easily and quickly rig a wide variety of CG characters with just a few clicks. Generate a flexible, modern character rig in minutes saving you days or weeks of work
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.
Fantasy Crocodile Rig is a free Maya rig that has a super appealing design.
Rocket Girl Rig is a free Maya rig from the VMComix animation blog.
Jasmine Rig is a free rig for Maya that has a load of great features!
Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!