ZephyrWest

WP Tag Manager aims to seamlessly integrate into your WordPress admin. It looks and feels like the rest of the admin. Install it and see what I mean.
Features:
<ul>
<li>Delete tags</li>
<li>Edit tags</li>
<li>Merge tags</li>
</ul>
Planned Features:
<ul>
<li>Tag splitting</li>
<li>Show untagged posts</li>
<li>Show similar tags</li>
</ul>

WP Tag Manager

An advanced tag manager that seamlessly integrates into WordPress.

Sam Margulies

Ever have one erroneously entered custom field name confuse all of your users and you just can’t figure out how to get rid of it? Delete Custom Fields will let you delete every instance of a custom field from your site. Once your remove a custom field using this plugin it will not appear in your list of custom fields when you edit a post or page.

Delete Custom Fields

Doesn't show all custom fields, though (even if you choose Show Hidden) - would be nice to have them as a list instead of a dropdown, but otherwise it does what it promises.

Works

Remove every instance of a custom field from your site.

WP Tag Manager vs Delete Custom Fields