There are times that you need to know the ID number of a page or post in WordPress and it can be more painful than it should be to find that number. This plugin does one simple thing: it displays the ID number of the page, post, category, or tag you are looking at in the WordPress Admin Bar. It appends this ID to the edit menu already on the Admin Bar. Needless to say, that means that this plugin is only useful for WordPress version 3.1 or above, since the Admin Bar was first introduced with 3.1.

Admin Bar ID Menu

Makes the ID number of the current page or post visible in the Admin Bar.

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.

Admin Bar ID Menu vs ShowID for Post/Page/Category/Tag/Comment