Built By Colossus Rigs are free and paid Maya rigs available for download. Founded and run by Roman Jeffery.
Built By Colossus Rigs
Live2D
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 ($25+)
M_Rigs are a set of dynamic Maya rigs available for purchase that ranges from a panther to a 6-armed humanoid.
Action VFX (FREE+)
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 Rigs (FREE+)
Josh Sobel has some great affordable rigs (his Kayla rig is especially great). You can also get his Bonnie rig for free!
Longwinter Rigs ($10/month)
Longwinter Studios are offering access to their whole rig collection (30+) for only $10 a month. Not too shabby!
Kiel Figgins Rigs (FREE+)
Kiel Figgins Rigs – An amazing selection of free and paid rigs (mostly paid) are available on Kiel Figgins’ website.
Animator's Resource Kit 