I install this plugin on every site I work on. It is so handy to have the ID listed next to the page or post. I don't understand why this has never become part of the WP base product?
On wordpress.org/plugins, there is a warning that this plugin has not been updated in over 2 years. It is working fine in WP 3.8.1. I suggest the author make a note in the plugin comments to recognize this fact, so that the plugin will show some activity. I think it's a keeper.