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

Darken-x

The plugin autogenerates the Dublin Core meta tags for all pages, posts and custom taxonomies, including Woocommerce products and categories. As a bonus, I also added the option to autogenerate Open Graph metadata, which can be turned on and off with a single click, and the additional option to have a default featured image for posts and pages that normally don’t have any featured image or for the situation where your main page is generated from a list of the latest posts or a simillarly custom-made solution.
Additionally, if the keywords meta tag option is active, it also has the capability of autogenerating keywords from post tags and titles (including Woocommerce products), join them with the general keywords as defined in the plugin settings, and then write them in the page’s “keywords” meta tag.
And lastly, it comes with a neat extra option that, if checked, cleans up your code by eliminating all useless meta tags and advertising comments such as the ‘Generators’ left by WordPress, WooCommerce, Slider Revolution, LayerSlider, WPML, WPBakery Page Builder (Visual Composer) and Yoast SEO. For each of them, there are various methods, plugins and bits of code that work to eliminate these comments, but I never saw them all in one place and within the same plugin, so I thought having an option to clean all metas, generators and comments added by other plugins with just one click would be a nice addition.
Very very lightweight and easy to use.

Dublin Core Metadata Generator

Version 1.0 released

Works great. Developer is very quick to respond when it comes to bug reporting. 

Does exactly what it says

A very lightweight plugin that adds the Dublin Core metadata to your WP website.

Custom Field Taxonomies vs Dublin Core Metadata Generator