UPDATE 1.0.5: Added the Sept 2012 GDR update. See infohere
After the release of Visual Studio 2010 there is one service pack (SP1). After that a series of updates have arrived, six in all. To find out which one you have installed, and also if there is any new ones you might have missed, can be hard, you need to know the KB numbers.
This tool gives you these answers. It tells you exactly which one you have, and also tells you what to install if you have missed one, including links to the download sites. Fortunately the updates in question are cumulative, so you only have to do one install for the updates, and one for the SP1 if you missed that one.
After installing the extension, go to the top level menu Help, and next to the bottom you find the Version Info.
For further information and details see thisblog post.
Minor update 15.8.2012: VS 2012 name change. Also supports VS Shell.