Workflow Inventory for the site collection
Provide an option to see the workflow inventory across the entire site collection, or from the current site and all sub-sites.
This is already available however to see it you need two things. 1. an enterprise license and 2. you need to be signed in with site collection administrator rights.
Also available is the overall Farm view, again this requires an enterprise license and central administration administrator credentials.
From both of these views one can filter to find all sub sites of a particular site or export as CSV and use something like Excel to perform more sophisticated filtering on any column.