Add custom class to a particular menu
Well, the code in tutorial that beth mentioned has bugs and does not work. The best solution I've found up to now is this:

&lt;?php
function YOUR_THEME_menu_tree($tree) {
  if (preg_match('/A unique string in your menu/', $tree)) {
      return '&lt;ul class="header-top-menu"&gt;' . $tree . '&lt;/ul&gt;';
  } else {
    return '&lt;ul class="menu"&gt;'. $tree .'&lt;/ul&gt;';
  } 
}
?&gt;


You can use any unique path or link title as string to identify the menu. This solution was inspired by a comment in the cited tutorial.