We just returned from a 3-week trip in central Europe (Budapest, Vienna, Prague, Krakow). Though it was a large-group tour it was very rewarding and fun. I wanted to recommend the best of many guides who helped us, Martina Cermakova in Prague. An excellent English speaker (majored in English in college), she knows a great deal about Czech history and architecture. She took us through Prague's Jewish quarter and Old Town one morning, then gave us a thorough and entertaining account of her country's history on a day-long bus ride to Brno and Krakow. Her website is www.praguetourguides.com and she offers small-group tours in Prague. Highly recommended.
This post reminds me that I am long overdue in recommending Katarina Kruspanova for private tours in Prague and the Czech and Slovak countryside. We spent a memorable 10 days with her in July 2006. Her email address is firstname.lastname@example.org. By the end of our trip with her, we felt like she was family. We also rented an apartment from her and it was a great value. Clean and away from the main tourist area, but convenient to sightseeing because of the trams.