<?php $tags = get_tags(); if ( $tags ) : ?>

  <select onchange="document.location.href=this.options[this.selectedIndex].value;">

    <option value="" selected="selected">タグを選択</option>

    <?php foreach ( $tags as $tag ): ?>

      <option value="<?php echo clean_url( get_tag_link( $tag->term_id ) ); ?>"><?php echo wp_specialchars( $tag->name ); ?> (<?php echo $tag->count; ?>)</option>

    <?php endforeach; ?>

  </select>

<?php endif; ?>