Tom Belknap

The ultimate auto-tagging plugin for WordPress.
NOTE: The plugin is now being maintained on GitHub, so please do contribute all your questions, comments, suggestions, fixes and improvements with me <a href="https://github.com/holisticnetworking/title-to-terms" rel="nofollow ugc">here</a>.
Title to Terms Ultimate is for every WordPress admin that needs tags, categories and other taxonomies filled out automatically. For each post type in your WordPress installation, T2TU will allow you to assign a taxonomy to be automatically updated with keywords pulled from the title of the post.
Every time a post gets saved or updated, T2TU analyzes the title of the post for usable keywords. A list of user-configurable “Stop Words” is checked by the parser, so that words like “I” or “going” don’t get made into ultimately useless tags.
<ul>
<li>Converts keywords in post titles to terms in any taxonomy</li>
<li>Includes user-editable list of words to be ignored.</li>
<li>Ignore list can be reset to default at any time.</li>
<li>Converts on save, not publish.</li>
<li>Configurable to either add new terms or append the list of terms.</li>
</ul>
Multisite admins take note: this plugin is especially helpful if you’re building a community-based site where tagging is important and your bloggers are not always diligent about tagging.
<h3>Acknowledgements</h3>
<ul>
<li>The WP Plugin Repository icon and banner were both made with the kind contribution of: <a href="https://thenounproject.com/search/?q=taxonomy&i=165760" rel="nofollow ugc">Muharrem Fevzi Çelik @ The Noun Project</a> (CC licensed)</li>
</ul>

Title to Terms Ultimate

The ultimate auto-tagging plugin for WordPress.

Amaury Balmer

WordPress 3.1 and up allow for reasonably simple custom taxonomy, this plugin makes it even simpler, removing the need for you to write any code.
Add support for taxonomy with or without hierarchical. Choose the selector type for write post.
Allow to add taxonomy to any objects registered on your installation. This plugin can be coupled with the plugin ACF to a new term meta database that allow to create on fly any field need for the taxonomy. For example, for a “Number” taxonomy, you can add fields : number ID, date of parution, screenshot, etc !
This plugin is developped on WordPress 3.5, with the constant WP_DEBUG to TRUE.
This plugin allows you to add taxonomy just by giving them a name and somes options in the backend. It then creates the taxonomy for you, takes care of the URL rewrites, provides a widget you can use to display a “taxonomy cloud” or a list of all the stuff in there, and it allows you to show the taxonomy contents at the end of posts and excerpts as well. 
To boot, it comes with a set of template tags for those who don’t mind adapting their own theme.
For full info go the <a href="https://github.com/herewithme/simple-taxonomy" rel="nofollow ugc">Simple Taxonomy</a> page.

Simple Taxonomy

WordPress 3.1 and up allow for reasonably simple custom taxonomy, this plugin makes it even simpler, removing the need for you to write any code.

Title to Terms Ultimate vs Simple Taxonomy