This is a copy of the famous sign that once stood at Checkpoint Charlie, a border crossing between East and West Berlin during the Cold War. The checkpoint remained in use until October 1990, when Germany was reunified. Checkpoint Charlie remains a major tourist destination, however, and a replica of the Allied checkpoint guardhouse was erected at the original site.
I visited Berlin for the first time in December 2009 and would highly recommend a trip there if you are interested in Cold War history. Despite the cold weather, I loved visiting during the winter. Flights to Berlin are relatively cheap at that time, and you can always warm up with some glühwein or jagertee.