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

pixelgrade

George Olaru

Andrei Lupu

With <a href="https://github.com/pixelgrade/pixfields" rel="nofollow ugc">PixFields</a> you can have a custom fields visual manager.
Note: this plugin does not add any style to the output of custom fields. Its just adding metadata and offers a way to overwrite the html from the theme. 
The theme should style the output of the metadata.

PixFields

PixFields offers you a nice interface to add custom metadata into your favorite post type.

Custom Field Taxonomies vs PixFields