This article explains a practical implementation of a technique outlined in the article "Sharing Drupal tables between databases using MySQL5 Views".
You have multiple (multisite) Drupal sites and you would like to manage the content for all of these sites through a single interface. Depending on the nature of a given piece of content, you may want the content published on one, several or all of your subsites, but you do not want to have to create copies of the same content for each site.
Taxonomy plus MySQL5 views. (NOTE: this solution will not work with versions of MySQL prior to 5.)