Fast IK is a Unity Inverse Kinematic solver. The main difference to all the other solvers out there is that the video shows how to use it and it is free.
Fast IK is based on the Fabic IK algorithm. It's super speedy and gives you nice IK animation. You can set different chain length and even poles. If you are used to blender's IK, you will get familiar with this right away.
The package contains a sample scene with three assets, that are going from extremely simple to advanced usage. You can use this for procedural animation.
If you find any bugs, just update the Github Project .
Yes, it is open source!