'Already Existing Tags' tries to make your life as blogger easier with the auto tagging function.
This plugin uses keywords provided by the user to automatically add tags to posts according to their title, content and excerpt.
The Official WordPress Fireplug-in. Allow your readers to get credit for visiting your website and reading your content.
This will automatically make tags with the tagthe.net and yahoo yql services when you save or update a post. Has the option for retagging all post.
Fast and efficient post tagging using multiple tag suggestion APIs, plus mass tag suggestions and editor.