Made this tool a while ago, but noticed it was missing a thread here.
Auto IKFK Switcher allows you to perform IK FK switching on almost any rig, after a simple setup process. It can also setup a scriptjob to perform automatic switching based on your control selection. This requires your rig to be able to show both IK and FK controls all the time.
For example, if you're animating in FK and want to switch to IK at some point, all you need to do is select IK control. This IK control will then snap to current FK bones, and keyframes will be added for the switching.
It also has a baking feature, allowing you to bake your animation from FK to IK or vice versa.
Feel free to report any bugs, will be happy to fix them.
Auto IKFK Switcher on Gumroad