You can put the following code in your theme’s functions.php file. If you’re using Thesis it would be in custom_functions.php. If your theme doesn’t have a functions.php file you can just make one (starting with <?php) and put it in the main directory of your theme. Or you can also create a simple plugin file and activate the code as a plugin.

function all_post_tags() {
$tags = get_tags(array('orderby' => 'name', 'order' => 'ASC', 'hide_empty' => 0, 'get' => 'all'));
	$tag_links = '<p id="alltags-post_tag" class="the-tagcloud"> ';
	foreach ($tags as $tag)
		$tag_links .= '<a href="#" rel="tag" class="tag-link-' . $tag->term_id . '">' . $tag->name . '</a> ';
	$tag_links .= '</p>';

?>
<script type="text/javascript">
	jQuery(function() {
		jQuery('p.tagcloud-link').after('<p class="alltags-link"><a href="#titlediv" class="alltags-link">Choose from all the Post Tags</a><?php echo addcslashes($tag_links,"'"); ?></p>');
		jQuery('p#alltags-post_tag').hide();
		jQuery('a.alltags-link').click(function() {
			jQuery("p#alltags-post_tag").toggle();
			return false;
		});
		jQuery("p#alltags-post_tag a").click(function() {
			tag_flush_to_text('post_tag', this);
			return false;
		});
	});
</script>
<?php
}
add_action('admin_head','all_post_tags');

I will investigate how to make this work again. Since the functionality is now partially built in, the method of posting the tags has changed a fair bit, but we should be able to work it.

I do know that get_tags() still works and gets an array of the tags and their details. I still use that function on my archive page.

What are you trying to do with this, given that WordPress has its own clicky list of tags on the write page?

To be honest, I’ve never fallen for ‘tags’. They’ve always bugged me, and organising them is a task in itself. As a result, I solely use categories.