Plugin library for Python: pluggy
I just added it to a backup runner project to give me an easy way to hook into the [start all]/[start one]/[end one]/[end all] points of the runner, without modifying the original project (besides adding the hook points).
So far, pluggy has worked very well. It's nice to have my scripts in a separate project, and quickly install an update once I have a plugin change.
It did require me to create installable projects (which has never gone well for me), but once I passed those hurdles, it worked nicely.