function widget_mytheme_kategorie() {
?>
<h2>Kategorien</h2>
<ul class="jens">
<?php wp_list_cats('sort_column=name&optioncount=1&hierarchical=1&'); ?>
</ul>
<?php
}
if ( function_exists('register_sidebar_widget') )
    register_sidebar_widget(__('Kategorie'), 'widget_mytheme_kategorie');