Having never heard of Stolperstein until I saw @fuzzyleapfrog's post, I looked it up in Wikipedia. And wow:
A Stolperstein (plural Stolpersteine) is a ten-centimetre (3.9 in) concrete cube bearing a brass plate inscribed with the name and life dates of victims of Nazi extermination or persecution. Literally, it means 'stumbling stone' and metaphorically 'stumbling block'.
The Stolpersteine project, initiated by the German artist Gunter Demnig in 1992, aims to commemorate persons at the last place that they chose freely to reside, work or study (with exceptions possible on a case-by-case basis) before they fell victim to Nazi terror, forced euthanasia, eugenics, deportation to a concentration or extermination camp, or escaped persecution by emigration or suicide.
As of June 2023, 100,000 Stolpersteine have been laid, making the Stolpersteine project the world's largest decentralized memorial.