We took this fantastic 4 hour Berlin tour with Insidertour (our guide was Brian Bell). We learnt so much about the history of Berlin. A lot of the history specialty museums are made free by German government because they want people to learn and remember the history (e.g the SSS/gestapo museum near finance ministry and the holocaust museum under the Holocaust Memorial).
Brian Bell was hands down the best tour guide we have ever had (the best ... not one of ...). His theater background made him a great story teller and his passion/love for this city showed through clearly. He was extremely knowledgeable too and answered any question with lots of historical artifacts. We were planning to leave the tour early because 4-hour is quite long but we stayed till the very end.