by Senff - a11n on WordPress.org
Simple Debug Info Panel shows a little box on your site with helpful debugging info for developers: post/page ID, which template is being used, etc.

Settings screen
Simple Debug Info Panel for WordPress will enable you to instantly see some “under-the-hood” debugging info for your site. With one click (or none at all), you can see which template is being used, what the post/page ID is, the current screen/viewport size, any categories a post is assigned to, and some more.
Every item’s visibility can be turned on or off, so you have complete control over what will be shown in the infobox.