DMG Categories Widget

Display categories as a list of links or a dropdown jump-menu (automatically navigate to page when selected).

Includes advanced options to change the taxonomy, display post counts and the category hierarchy, change how they are ordered, add CSS styles and modify the title.

Replaces the built in Categories Widget (WP_Widget_Categories).

A hook is available to filter the title: dmg_categories_widget_title.

For example, to change the title on a single page or post, you could add this to your functions.php file:

function myTitleFilter( $title ) {     if( is_singular() )     {         return "<strong>$title</strong>";     }     else     {         return $title;      
    } } add_filter( 'dmg_categories_widget_title' , 'myTitleFilter'); 

More information about this plugin can be found at http://dancoded.com/wordpress-plugins/categories-widget/.