One of the world's smallest bars, American Bar resides in none other than central Vienna. After a visit to the United States, legendary architect Adolf Loos was inspired to design the tiny bar, which opened in the early 1900's as a private men's club. Today, the smoke-filled black onyx and dark marble interior teems with trendy singles. Also known as the Loos Bar, this Viennese hot spot features sleek decor and a mirrored wall, which appears to enlarge the otherwise cramped space. Although taking photos is tempting, an intimidating sign out front notes that photography is strongly discouraged.