Take the tedious labor out of updating multiple posts with the same tag by using the Mass Add Tags plugin. This small, two-file plugin takes up almost no space on your server and creates a single menu item. It’s simple and straight-forward. Enter the tag you want to add, enter the character or phrase that must be in the title for the phrase to be added and click go. You’ll be notified of how many, if any, posts were updated.
Download Mass Add Tags for WordPress.
Current release: 1.2
Corrected typographical errors, streamlined certain portions and updated styling.
Features anticipated for future: add multiple tags at once, search post body, search existing tags, apply tags to all posts in a category.