How do I create relative menu links?
When you add a new link, the link is validated from menu_edit_item_validate(), which calls drupal_valid_path() to check if the path is valid, or it can be accessed from the user.
drupal_valid_path() returns TRUE if 


The path is &lt;front&gt; (&lt;front&gt;/directory, or similar paths are excluded)
The path is an external path (the function url_is_external() returns TRUE)
The path is associated with a menu callback


Apart from using absolute URLs, the only solution is to have a custom module that defines a menu callback associated with the path you want to use.
When you use the URL that points to a file, or a directory, the web server should show that file or directory, without invoking Drupal. That is what happens with Apache, as the .htaccess file that comes with Drupal contains the following directives:

# Pass all requests not referring directly to files in the filesystem to
# index.php. Clean URLs are handled in drupal_environment_initialize().
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !=/favicon.ico
RewriteRule ^ index.php [L]


In your case, the custom module should contain code similar to the following:

function mymodule_menu() {
  $items = array();

  $items['the_folder_for_the_other_content/%menu_tail'] = array(
    'title' =&gt; 'Folder content', 
    'page callback' =&gt; 'mymodule_view', 
    'access callback' =&gt; TRUE,
    'load arguments' =&gt; array('%map', '%index'), 
  );

  return $items;
}


You can use any values for title and page callback, as that menu callback should never be called. The value of access callback says to Drupal that every user has access to that menu callback.

As an alternative you can define a menu callback associated to "alias_for_directory/%menu_tail" that redirects the user to the directory you want they see (using drupal_goto()). I would use this solution when the server does not use the content of .htaccess, and it would invoke Drupal even when the URL is pointing to an existing file or directory.