Display All Post Types in Category Archives

If you want WordPress category archive pages to display all posts regardless of post type, add the following code to your functions.php file:

function any_ptype_on_cat($request) {

    if ( isset($request['category_name']) )
        $request['post_type'] = 'any';

    return $request;
}
add_filter('request', 'any_ptype_on_cat');

For more on custom post types, see How to Create Custom Post Types.