I find myself in every recent version of WP (and WPMu) searching for the ID numbers for various pages, posts, categories, and/or tags on a number of occasions. I’m not sure when this feature got dropped from the core, but I do remember it was at one time part of the edit post/edit page metadata. As of the latest versions the only way to find ID numbers is to actually edit the page (or post, tag, category, comment, etc.) and look for the ID value in the URL field (pictured below). Not very intuitive, in fact, ridiculous.
ShowID for Post/Page/Category/Tag/Comment plugin actually shows the ID value right from the edit section of any of these elements. Something like the following:
I am going to activate this plugin sitewide for UMW Blogs, because a number of the plugins/widgets we have actually ask for page/posts IDs to exclude pages/posts etc. from a widget, or include categories in a page. And explaining how to find this information without this plugin is just that much more difficult. I really don’t understand why this isn’t a core feature of WP given core widgets (such as the pages widgets) and a wide variety of plugins depend on ID values.