Free Sofia Tour, run by an NGO, is one of the best attractions in Sofia. Enthusiastic young people will walk you through the highlights of central Sofia with many an interesting tale to tell. Starting point is the Palace of Justice, corner of Alabin St. Look out for the tour guides with their signs. Allow at least 2 hours. No reservation or tickets are required, just show up -  there is no fee, though if you are satisfied with your guide you are invited to leave a gratuity to this non-profit organisation.
For larger groups it is advisable to contact them to set up your individual tour.
They also offer a variety of special interest tours, including the popular Communist walking Tour, for which the prices are between 18 - 24 leva per person.