ProRigs are Maya rigs developed by talented artists from animation studios such as Disney, Dreamworks, Pixar and Sony. Available on a subscription model which gets you access to over 40 rigs.
ProRigs
Ystra – Astral Witch
Ystra – Astral Witch is a free Maya rig by Christoph Schoch based on the concept by Alexis Pflaum. Download it HERE
Animation Methods
Animation Methods is a website hosted by animator David Rodriquez and has links to hundreds of rigs, tools and tutorials.
League of Legends Rigs
League of Legends Rigs have been released for the recent AnimChallenge contest. All rigs available for Maya (Agora). Braum and Annie are also available for Blender (RiggingDojo)
Built By Colossus Rigs
Built By Colossus Rigs are free and paid Maya rigs available for download. Founded and run by Roman Jeffery.
Agora Studio Rigs
Agora Studio have been creating a bunch of Maya rigs and releasing them for free primarily to be used in the AnimChallenge. Check them out HERE
Grease Pencil Rigs
Grease Pencil Rigs are a free set of basic rigs for Blender 2.8 that are for use with Grease Pencil. They are aimed to ease the drawing of characters by showing construction lines which you can use for reference while drawing.
Red Dragon Rig
Red Dragon Rig is a free Maya rig made by junior rigger, Elise Mathez
Tre Vital Rigs (FREE)
Tri Vital has created a collection of awesome Maya rigs and put them on gumroad for free!
Animator's Resource Kit 