scribu

Custom fields are great for storing various meta data about a post. However, if you routinely need to display posts that have a certain custom field value, you should consider switching to a <a href="https://codex.wordpress.org/Custom_Taxonomies" rel="nofollow ugc">custom taxonomy</a>, which is better suited for grouping posts.
With this simple plugin, you can convert all custom fields with a certain key to terms in a certain taxonomy, while maintaining the post association.
Note: If you’re looking for the old version, see <a href="https://wordpress.org/extend/plugins/custom-query-fields/" rel="ugc">Custom Query Fields</a>
Links: <a href="http://scribu.net/wordpress/custom-field-taxonomies" rel="nofollow ugc">Plugin News</a> | <a href="http://scribu.net" rel="nofollow ugc">Author’s Site</a>

Custom Field Taxonomies

Convert custom fields to tags, categories or taxonomy terms

vlad.olaru

pixelgrade

George Olaru

Andrei Lupu

Razvan Onofrei

cristianfrumusanu

With <a href="https://github.com/pixelgrade/pixtypes" rel="nofollow ugc">PixTypes</a> you can allow your theme to define what custom post types or meta-boxes should be active when your theme is up
Note: This plugin is addressed to developers, it doesn’t do nothing if it isn’t <a href="https://github.com/pixelgrade/pixtypes#pixytpes_config" rel="nofollow ugc">properly configured</a>.

PixTypes

A WordPress plugin for managing custom post types and custom meta boxes from a theme.

Custom Field Taxonomies vs PixTypes