Many people have asked me what is the most photographed place of the world at Panoramio. I can't check the whole planet, but after some searches I believe that Saint Peter Square in Rome is the place where more people have mapped photos. The second place is Tour Eiffel, followed very closely by Piazza San Marco in Venice and the Roman Colosseum. Italy has three places in the first four positions, it doesn't look like a coincidence that Italy is the country with more places in the World Heritage list from Unesco.
Other places that have been very much photographed are Statue of Liberty in New York (USA), the Opera House of Sydney (Australia), Hagia Sophia in Istanbul (Turkey), London Eye (UK), Alhambra Palace in Granada (Spain), Inca Ruins in Machu Picchu (Perú), Athenian Acropolis (Greece), Church of Sagrada Familia in Barcelona and Niagara Waterfalls.