Term Debt Consolidator
As posts are created and assigned Categories and Tags over time, the number of terms can grow quite large. Without careful curation, a site can wind up with multiple terms that mean the same thing. Instead of organizing site content and making posts in every topic easily findable, multiple terms for the same basic subject can fragment the site and reduce usability.
Term Debt Consolidator is a WordPress plugin written by INN that can analyze your WordPress site's Categories and Tags (and optionally terms in custom taxonomies) to identify terms that might be redundant. It will then suggest terms for consolidation and deletion to improve the effectiveness of WordPress's taxonomy functionality.
For full documentation see the Term Debt Consolidator project docs.