Built By Colossus Rigs are free and paid Maya rigs available for download. Founded and run by Roman Jeffery.
Live2D has a bunch of software that allows 2D artists to create animations from their artwork. Available in both FREE and PRO versions.
M_Rigs are a set of dynamic Maya rigs available for purchase that ranges from a panther to a 6-armed humanoid.
Action VFX has a collection of free VFX that are useful for adding post effects to your animations. There is also a much larger collection available for purchase.
Josh Sobel has some great affordable rigs (his Kayla rig is especially great). You can also get his Bonnie rig for free!
Longwinter Studios are offering access to their whole rig collection (30+) for only $10 a month. Not too shabby!
Kiel Figgins Rigs – An amazing selection of free and paid rigs (mostly paid) are available on Kiel Figgins’ website.