During some of the work on views we came to the conclusion that it would be helpful to find out which module provides what kind of plugins.
This is just pseudo-code but for example plugin_fetch_plugin_types('aggregator', 'aggregator') would return array('fetcher'), plugin_fetch_plugin_types('ctools', 'views') would probably return array('style', 'field') one day.
Do you think such a function would be helpful in general?