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.
Alien FPS Rig is a free first person rig for Maya based on a sci-fi theme (inspired by Alien). Made by Guillermo Moreno Alfaro.
Epic Pen allows you to draw directly onto your screen, making it perfect for leaving drawovers, notes and visual feedback on animations
Louise is a free high res facial rig for Maya, Unreal and Unity with a complete set of 236 blendshapes
Animate With Arran is a YouTube channel by Arran Baker who has recently started to post some great free videos around animation process, workflow tips and more.
Animation Methods is a website hosted by animator David Rodriquez and has links to hundreds of rigs, tools and tutorials.