It is doubtless that the World War II and the Jewish Holocaust was a dark time of History that should not be forgotten; on the contrary each of us should remember it forever as a lesson of what mankind can horridly do. For that you have plenty of historical material available online and printed publications and books used in high-school and college.
But sometimes it’s just not enough and if you don’t have the chance to visit some scary concentration camps, by the way very well-preserved, in Europe I would suggest you visiting ScrapBooksPages.com, a very comprehensive and illustrative site with featured photographs of Dachau, Auschwitz and Theresienstadt. To be objective, the site is more touristic than historical, if you can believe that someone can speak about Holocaust Tourism but sadly it is possible. Nevertheless it is a well-suited site with pictures from the camps, the barracks and Holocaust memorials. The pages get frequently updated with new material and photographs. A site not only of Jewish interest but general interest with details about German and other European cities and very interesting stories such as Anne Frank House, a very worldwide famous journal of someone who sadly lived the Holocaust from inside. 
