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
FPS Rig (and Gun)
FPS Rig (and Gun) is a free maya rig by Mattias Kirsten that includes arms, hands and a gun. Grab it from the bottom of his personal website
Alien FPS Rig
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.
FPS Rig (AK47)
FPS Rig (AK47) is a free First Person maya rig that includes the hands, camera and gun. Grab the files from this google drive folder by Haydn Cooper
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.
BOTW Link Rig
BOTW Link Rig is a free Maya rig based on the Breath Of The Wild version of Link (similar to the previously posted Zelda Rig). Made by Christoph Schoch.
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
BOTW Zelda Rig
BOTW Zelda Rig is a free Maya rig based on the Breath Of The Wild version of Zelda. Made by Christoph Schoch.
Animator's Resource Kit 