I was looking for some way of adding a selectbox in the navigation so that one could pick a category and then get directed to the page with all products in that category. After much looking I found this page, that did sort of what I wanted. Customizing I got the following code for the top block in Magento.
<div class="header-nav-container">
<div class="header-nav">
<select class="browser_box" name="browser" onchange="location = this.options[this.selectedIndex].value;">
<option>Please select a category...</option>
<?php
$obj = new Mage_Catalog_Block_Navigation();
$store_cats = $obj->getStoreCategories();
$current_cat = $obj->getCurrentCategory();
$current_cat = (is_object($current_cat) ? $current_cat->getName() : '');
foreach ($store_cats as $cat) {
echo '<option value="'.$this->getCategoryUrl($cat).'">'.$cat->getName()."</option>";
}
?>
</select>
</div>
</div>