Custom module with custom theme/template
As Clive said, you can simply return HTML instead of the render array, e.g. 

function myApp_theme() {
    $html = "&lt;html&gt;&lt;head&gt;&lt;/head&gt;&lt;body&gt;Hello World&lt;/body&gt;&lt;/html&gt;;      
    return $html;
}


A more general way is to modify the page.tpl.php in your theme and add a wrapper around the normal styling.  E.g.

&lt;?php if ( $plain_page ) { ?&gt;
  &lt;?php print render($page['content']); ?&gt;
&lt;?php } else { ?&gt;
Normal stuff
&lt;?php } ?&gt;


Then in your module, use the template_preprocess_page hook to set the $plain page variable.  E.g.

my_module_preprocess_page( &amp;$vars ) {
  [ check if it's a page you want as plain ]
    $vars['plain'] = TRUE;


You will only need the content for the  tags as the html.tpl.php template handles that.