Source: https://github.com/volvis/BlenderScripts/blob/master/CopyTransform.py
Super simple addon for storing and retrieving (or copy/pasting) transformations. Works with both bones and objects.
I mainly use it for aligning objects and setting the entry and exit keys for the Child Of constraint. (ie. when influence drops to zero, retain the position the object had when it was one… I believe there was an addon for that too, though.)