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.
WP Auto Tagging WordPress Plugin Will Automatically Generate & Add Tags to WordPress Posts.
Looks for already existing tags within your posts.
Fast and efficient post tagging using multiple tag suggestion APIs, plus mass tag suggestions and editor.
Imagga Auto Tagging is a plugin that automatically adds tags to your posts based on their featured images.
The Official WordPress Fireplug-in. Allow your readers to get credit for visiting your website and reading your content.