https://blendermarket.com/products/rig-ui

     This is a highly configurable and dynamic system that will allow riggers to configure and make work almost any IK/FK system reliably.

The setup is as complex as the system is complete. But the basic mechanism is simple to understand, it will use pairs of bones to match certain transforms between them when swapping between IK and FK modes. This allows the limbs to perfectly match each other when changing between modes.