Rig on the Fly is a Rigging and Animation system. It is a bit of a paradigm shift in how we approach 3D animation and can be extremely powerful.
Downloads:
https://dypsloom.com/Assets/RigOnTheFly/
https://gitlab.com/dypsloom/rigonthefly
Doc:
https://gitlab.com/dypsloom/rigonthefly/-/blob/master/README.md
An interview explaining some of the underlying principles and ideas behind Rig on the Fly: