I joined the Sandeman's Tour in both Prague and Berlin and had a fun and educational time. My tour guide in Prague was an Irish lad named Declan. Declan was very knowledgeable and had a flare for the dramatic, which made his history lessons more like story-telling. It was fantastic! I actually remember thinking to myself that I felt sorry for the people in other walking tours that we passed along the way. Almost all of the walking tours in Prague leave from the Old Town clock. If you see Declan in his red Sandeman's uniform I would highly suggest that you follow him around for the next three hours.
The tours in Berlin meet at the Brandenburg gate and take you to a number of significant sites. Perhaps the most interesting things about this tour is that you get to see the history of Berlin layered on top of itself as things have been destroyed and rebuilt many times over the course of history. For example, there's a protected piece of the Berlin Wall that sits on top of the old Gestapo headquarters. The area has been excavated so that you can view both pieces of history at the same time.
And well, by now you probably know that "free" doesn't always mean what you think it means. These tours aren't so much free as they are "pay-what-you-can". At the end of the tour you are expected to tip your guide. There is no set amount so you can use your own budget and enthusiasm for your tour to determine how much you'd like to give.