Extension:CategoryIntersection

The CategoryIntersection extension provides a special page - Special:CategoryIntersection.

You put in a list of categories to this page (separated by newlines), and the extension returns the articles that are in all the categories listed.

It stores hashes of what categories a page is in in a separate table in db to (I assume) make things efficient.

Somewhat akin to the DynamicPageList extensions (but probably much more efficient).

Installation
To install this extension, do the following: