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.

Luis Rock

Master IDs plugin offers an easy way to find any post type ID. You just need to choose the post type wanted (page, post or any custom post type) and you will see all posts related, with the their respectives Titles and IDs, in a dropdown list.
Master IDs is also great for showing how many posts you have under each Post Type.

Master IDs

Find any post type ID (Post, Page or CPT) in a dropdown list.

ShowID for Post/Page/Category/Tag/Comment vs Master IDs