antonioandra.de

Enables your blog readers to suggest new post tags or upvote/downvote existing ones. 
Downvoted tags are eventually removed. 
A few more features are on the works as is the expansion of this readme document.
<h3>RoadMap</h3>
Add some frontend feedback; 
Add support for custom taxonomy tagging; 
Synonym dictionary; 
Better documentation, ui and automatic integration.

Author: António Andrade

Enables your blog readers to suggest new post tags or upvote/downvote existing ones.

dangrossman

The Calais Archive Tagger plugin automatically goes through your archives and tags every post you’ve written. The plugin uses the Open Calais API to perform semantic analysis of your post text and suggest tags. If a post already contains a suggested tag, that tag isn’t added, but other new tags found are. It takes about 5 minutes to tag 200 posts.
This plugin requires PHP 5 and the cURL library (both of which are available on most web hosts).
Also see <a href="https://wordpress.org/extend/plugins/calais-auto-tagger/" rel="ugc">WP Calais Auto Tagger</a> for suggesting tags as you write new posts.

WP Calais Archive Tagger

Goes through your archives and adds tags to your posts based on semantic analysis.

Author: António Andrade vs WP Calais Archive Tagger