quick & dirty hack to put a Tags dropdown on the "All Posts" page in WordPress admin.

gistfile1.php

add_action('restrict_manage_posts','my_restrict_manage_posts');

function my_restrict_manage_posts()
{
  global $typenow, $wp_query;

	if ($typenow=='post')
	{

		$current_tag = $wp_query->query_vars['tag'];

		$tags = get_terms('post_tag');

		?>
		<select name="tag">
			<option value="">All Tags</option>
			<?php

			foreach( $tags as $tag )
			{
				echo '<option value="' . $tag->slug . '"';

				if( $tag->slug == $current_tag )
				{
					echo ' selected="selected" ';
				}


				echo '>' . $tag->name . '</option>';
			}

			?>
		</select>
		<?php
	}
}