The US Tagging community project on Stamp Smarter has undergone an even larger and more significant update since Mark's post (above) a few weeks ago.
New content, including many tagging image examples, has been added thanks to contributions from Hans van Gils (translation done by Mark). There is also a new responsive 'tabbed' interface. Mark has worked tirelessly on the database; it now has over 10,275 tagged stamps listed. I believe that this is the largest, most complete US tagged stamp reference available anywhere. You can see the update page here, http://stampsmarter.com/features/Tagging_Home.html
Note that you may need to force your browser to refresh (typically F5 key) to fetch the revised code.