Stumbled across this website just this morning. A few of the places I have visited are considered historical sites and have markers placed.
Pretty easy to navigate, pick a country, then a city and it will list the historical markers for that location. Often with photos if they have been contributed.
Clicking on the location will bring up a map, address, and any additional photos.
Note that I found the map feature to be unreliable, but the addresses I checked in google earth were dead on.
There are about 100 or so in my home town missing photos. I now have a new mission!!! Historical Markers Website