Free tours of Vienna
The tours are charged for at the end - that's how the new "Free Tours" of the city work. Certified tour guides wait at fixed meeting points for tourists who'd like to go on a tour of the city. Most of the various walking tours lead to the most important sights in downtown Vienna and give a good initial overview of the city. The guide leads the tour in return for tips. That means the participants give the amount they deem appropriate at the end of the city tour. If they didn't enjoy it, they don't have to pay a cent.