水野史土

This plugin shows post/page/category/tag/comment/media/user IDs on admin’s edit post/page/category/tag/comment/media/user pages. You can see the ID when you visit an edit post/page/category/tag/comment/media/user page. 
Since this plugin hooks on ‘edit’ link, only users with priviledges can see IDs. For example, a user of author role can see post IDs, while a user of subscriber role cannot.
You can find a brief description in Japanese on a book <a href="https://php4wordpress.calculator.jp/" rel="nofollow ugc">Introduction to PHP for WordPress Users.</a> (written in Japanese.)

ShowID for Post/Page/Category/Tag/Comment

This plugin shows post/page/category/tag/comment/media/user IDs on admin's edit post/page/category/tag/comment/media/user pages.

echoplugins

This plugin shows (reveals) IDs on admin screens that list posts, pages, custom post types and taxonomies, and others.
You can choose to display the ID in a new ‘ID’ column and/or under each row title after the ‘Edit | Quick Edit | View’ actions.
<h4>Features</h4>
<ul>
<li>
Choose where to display IDs:
<ul>
<li>in a new ‘ID’ column</li>
<li>under each row title beside the ‘Edit | Quick Edit | View’ actions</li>
</ul>
</li>
<li>
Reveal IDs in the following lists:
<ul>
<li>Posts</li>
<li>Pages</li>
<li>Categories</li>
<li>Tags</li>
<li>Comments</li>
<li>Custom post types</li>
<li>Custom taxonomies</li>
<li>Users</li>
<li>Media</li>
</ul>
</li>
</ul>
<blockquote>
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</blockquote>

Show IDs by Echo

Show IDs on admin pages for posts, pages, categories, taxonomies, custom post types and more.

ShowID for Post/Page/Category/Tag/Comment vs Show IDs by Echo