How do I insert HTML onto every page?
You probably want hook_page_build():

function slidedown_page_build (&amp;$page) {
  $path = drupal_get_path('module', 'slidedown');

  drupal_add_js($path . '/slidedown.js');
  drupal_add_css($path . '/slidedown.css');

  $page['page_top']['slidedown_template'] = array(
    '#weight' =&gt; -1000, 
    '#markup' =&gt; theme('slidedown_template'),
  );
}


I can't remember if that hook is too late to add JS and CSS.  You may need to put those in a template_preprocess_html().  That is a better choice that hook_init().