Mod Articles Count
A module extension for the Joomla 1.5, 2.5 and 3 CMS. The articles count module (as the name suggests), counts the articles in your Joomla categories, and displays them in a list with a link to the category. Also there is a version available which counts K2 articles in categories in exactly the same way.
Optionally the module can use a collapsible tree to show the category structure, as shown below:-
This is a free module released under the GPL, the download links are below.