A quick google search gives this:
http://code.google.com/p/django-treemenus/
http://code.google.com/p/django-menuse/
You can also create such a simple menu manually, just go to the template for the list of menu items, the active menu and the list of submenu items for the active menu and the active submenu item:
<ul>
{% for item in menu_items %}
<li>
{% if item.id == active_menu_item %}
<span class="active-menu-item">{{ item }}</span>
<ul>
{# Similar code for submenu items #}
</ul>
{% else %}
<a class="inactive-menu-item" href="{{ item.url }}">{{ item }}</a>
{% endif %}
</li>
{% endfor %}
</ul>