Sandemans New Europe Tours: They offer a daily Classic Berlin Free Tour that covers major historical highlights, including Brandenburg Gate, Checkpoint Charlie, and the Holocaust Memorial. Guides work on a tips-only basis. 

Tours usually start at Pariser Platz near the Brandenburg Gate at 10 AM, 11 AM, and 2 PM. 

Reservations are recommended, but there’s no upfront cost.

Generation Tours: This tour explores Berlin’s transformation from a Prussian empire capital to the modern city it is today. 

Their guides focus on historical narratives, including World War II and Cold War landmarks, and they operate on a tips-only basis. 

Tours meet at Brandenburg Gate and typically run daily.

Alternative Berlin Tours: For those interested in more recent history, these tours present Berlin's countercultural scenes, including graffiti, squatting movements, and the fall of the Berlin Wall. 

The tours are also free but operate on a tip basis.

For all these tours, while they are free, tipping is customary if you enjoy the experience. 

Be sure to check the respective websites for updates on timings and meeting points before attending.