Add conditional javascript from CDN (external library)
The Boilerplate theme does it for CSS by adding to template.php in the theme:

function boilerplate_preprocess_html(&amp;$vars) {
  if (!module_exists('conditional_styles')) {
    boilerplate_add_conditional_styles();
  }
}

function boilerplate_add_conditional_styles() {
  // Make a list of base themes and the current theme.
  $themes = $GLOBALS['base_theme_info'];
  $themes[] = $GLOBALS['theme_info'];
  foreach (array_keys($themes) as $key) {
    $theme_path = dirname($themes[$key]-&gt;filename) . '/';
    if (isset($themes[$key]-&gt;info['stylesheets-conditional'])) {
      foreach (array_keys($themes[$key]-&gt;info['stylesheets-conditional']) as $condition) {
        foreach (array_keys($themes[$key]-&gt;info['stylesheets-conditional'][$condition]) as $media) {
          foreach ($themes[$key]-&gt;info['stylesheets-conditional'][$condition][$media] as $stylesheet) {
            // Add each conditional stylesheet.
            drupal_add_css(
              $theme_path . $stylesheet,
              array(
                'group' =&gt; CSS_THEME,
                'browsers' =&gt; array(
                  'IE' =&gt; $condition,
                  '!IE' =&gt; FALSE,
                ),
                'every_page' =&gt; TRUE,
              )
            );
          }
        }
      }
    }
  }
}


Then added to the .info file:

stylesheets-conditional[lte IE 8][all][]  = css/compiled/ie8.css
stylesheets-conditional[lte IE 7][all][]  = css/compiled/ie7.css
stylesheets-conditional[lte IE 6][all][]  = css/compiled/ie6.css


I'm sure this could be easily adjusted for JavaScript instead