Louise is a free high res facial rig for Maya, Unreal and Unity with a complete set of 236 blendshapes
Sam & Amy Rigs are another great set of rigs made by Gabriel Salas
Henry rig is a free rig made by students from Cogswell University
AnimProps is a website where 2D and 3D artists can create an account for free to share, sell or purchase artwork. A big resource for animation rigs.
Built By Colossus Rigs are free and paid Maya rigs available for download. Founded and run by Roman Jeffery.