Rezerla

Rezerla Post List Field Search adds a Search in dropdown next to the search box on post list screens (<code>edit.php</code>). Search by title, content, slug, custom fields, post meta, or taxonomy — individually or across all configured fields at once.
<ul>
<li>Admin-only — no impact on front-end search</li>
<li>Per post type configuration</li>
<li>Core fields, post meta, and taxonomies</li>
<li>Optional list table columns (Show in table checkbox)</li>
<li>Temporary column and highlight when searching by a single field</li>
<li>Field discovery for meta keys and taxonomies</li>
<li>Lightweight — no frameworks or subscriptions</li>
<li>Default configuration for WP Store Locator (<code>wpsl_stores</code>)</li>
<li>No data collection or external API calls</li>
<li>Translations: English (default), Polish, German, French, Spanish, Italian</li>
</ul>
<h3>Usage example</h3>
On a WP Store Locator store list (<code>edit.php?post_type=wpsl_stores</code>):
<ol>
<li>Enter “Katowice” in the search box.</li>
<li>Select City in the dropdown.</li>
<li>Click the search button.</li>
</ol>
URL: <code>edit.php?post_type=wpsl_stores&s=Katowice&rezerla_plfs_field=city</code>
<h3>List table columns</h3>
Check Show in table in field settings to display meta or taxonomy values on the list. 
When searching by a single field, that column appears temporarily and is highlighted even without the checkbox.

Rezerla Post List Field Search

Adds a configurable "Search in" dropdown to WordPress admin post lists for searching by title, content, slug, meta, and taxonomies.

Automattic

Daniel Bachhuber

A quick and easy way to see post meta associated with any given post. When activated, this plugin prints your post meta keys and values inside of a post meta box.
Originally developed for <a href="http://vip.wordpress.com/" rel="nofollow ugc">WordPress.com VIP</a>-hosted clients, we thought it would be useful for development environments, etc. too. Feel free to <a href="https://github.com/Automattic/Post-Meta-Inspector" rel="nofollow ugc">fork the plugin in Github</a> and pull requests are always welcome. Hit us with feedback, questions, bug reports, and feature requests in the forums.

Post Meta Inspector

Lovely little tool. Saves having to go into the database and look through the postmeta table when debugging. Thanks!

Rezerla Post List Field Search vs Post Meta Inspector