Admin Bar ID Menu
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.
Get the source on Github.