Philip Newcomer

This plugin makes all custom fields (post meta) visible in the “Custom Fields” meta box on post edit screens.

Show Hidden Post Meta

As a plugin developer, I create hidden post meta all the time and sometimes I need to see it to debug it. This does what it says it does, and well. Its simple and hasn't been updated for a while, but I'm glad it exists!

PS: If you need someone to take this one over, let me know. I'll keep it tested and updated if you're unable to.

Still works on 5.5

Makes hidden post meta visible on post edit screens

Alex Prokopenko / JustCoded

Just Custom Fields adds ability to extend your Posts, Pages (and other custom post types) and Taxonomies with additional fields. After installation, you will see a simple settings page, which is self-explanatory to use. 
We use the standard WordPress PostMeta API and TaxonomyMeta API to save fields data, so you can use standard WordPress functions in your themes/plugins to get data.
IMPORTANT We do not recommend update plugins on your existing sites from version 2.* to version 3.0+. 
JCF v3.0+ is not compatible with versions 2.*, so some field settings can be lost (and as a result – wrong values on post edit pages).
Plugin supports such field types:
<ul>
<li>Input text</li>
<li>Select Box</li>
<li>Multiple Select Box</li>
<li>Checkbox (single and multiple)</li>
<li>Textarea (you can use WordPress editor too)</li>
<li>Datepicker</li>
<li>Simple Media (files and images upload)</li>
<li>Table</li>
<li>Fields Collection (repeatable fields groups)</li>
<li>Related Content (to set a relation to another Post/Page or Custom Post Type)</li>
</ul>
You can read full documentation on our site: http://justcustomfields.com.
For quick demo you can watch our presentation video: 
https://www.youtube.com/watch?v=7KJeH2d_v48
Starting from v2.0 we have different options to save plugin configuration:
<ul>
<li>Ability to set Fields Settings global if you have MultiSite. So you can set them once, without copying all settings to every new site.</li>
<li>Ability to save Fields Settings to a file system (inside theme or wp-content folder). We expect this option will be popular among the developers. It will be much easier to move your fields’ settings between site versions (dev/production).</li>
</ul>
Starting from v2.2:
<ul>
<li>Simple Media field is now a single file only (before it was multiple). It uses new media upload box from WordPress and saves post thumbnail ID (before it was just a file url). So now you can use the get_the_post_thumbnail() function to work with attachments.</li>
<li>All fields now have “Shortcodes” and hints how to use them inside the templates for non-professional developers.</li>
</ul>
For easy migrations between different sites, we have Export/Import options.
FEEL FREE TO CONTACT ME IF YOU FIND ANY BUGS/ISSUES!
ISSUES TRACKER 
Plugin code is open source and placed in the GitHub public repository. I listed known bugs and future features there. You can post new bugs or feature requests for me there. 
https://github.com/aprokopenko/justcustomfields/issues

Just Custom Fields

Turn WordPress into more powerful CMS by adding advanced and easy to use custom fields

Show Hidden Post Meta vs Just Custom Fields