John LeBlanc

Assigns tags to posts containing specific text strings, handy for filtering within the loop. A gift to <a href="http://twitter.com/docrock/status/58054643929985024" rel="nofollow ugc">@docrock</a> from <a href="http://twitter.com/johnleblanc/status/58303307713753088" rel="nofollow ugc">@johnleblanc</a> to help keep <a href="http://instagr.am/" rel="nofollow ugc">Instagr.am</a> image posts out of the <a href="http://www.mrdocrock.com" rel="nofollow ugc">mrdocrock.com</a> homepage loop. Born April 13, 2011 in <a href="http://maps.google.com/maps?ll=21.408966,-157.831993&spn=0.091894,0.162907&t=h&z=13&lci=com.panoramio.all,com.youtube.all,com.google.webcams,org.wikipedia.en" rel="nofollow ugc">Kaneohe, HI</a>.

Doc’s Auto-tags

Assigns tags and/or categories to posts containing specific text strings, handy for filtering within the loop.

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.

Doc’s Auto-tags vs Simple Taxonomy