A web site project documents the traces on doorposts in Poland where mezuzahs were once affixed — and asks people to send in their photos, with location noted.
All over Poland (and other countries, of course) many doorposts still bear grooves, paint discolorations, nail holes and other scars that show where a mezuzah had been affixed before WW2 when Jews had lived in the house. These are very poignant, and even dramatic, signs of the Holocaust.
So far, the site has collected images from about two dozens town, cities and villages. They are posted with the address, date of photograph and name of photographer, as well as other information in some cases.