UPDATE: One day in July 2019, I came across a small ceremony, where the artist was laying down some Stolpersteine:

This very moving initiative by the artist Gunter Demnig to commemorate individuals and often entire families wiped out during the Holocaust – Stolpersteine can be found all over Berlin (over 5000), but also in other cities such as Cologne, Rome, …. Read about them here.: http://www.stolpersteine.eu/en/. And read about them in this book: Stolpersteine in Berlin – 12 Kiezspaziergänge. I bought mine at Dussmann das Kulturkaufhaus which, incidentally, in addition to being a huge bookshop including a large section of books in English, has an excellent selection of classical CDs and DVDs.

The app ‘Stolpersteine’ is a great help when walking around Berlin. As you will see below, I always stop and take a photo whenever I spot one or more of them.