Minimal Python plugin mechanism
Check out how Django apps work:


Plugins are genuine Python packages, so the standard tools can be used to install them
Plugins that are actually used must be in the PYTHONPATH and listed (only the package name) in the configuration.
There are scripts to help create, test, ... plugins
The plugin structure is extensible in all possible ways.
Plugins are reloaded when they change in development mode.


I'm not saying that you should use Django if you're not doing web development, but you can inspire your plugin system on it.