Great Wall of China. A search of the Lexis-Nexis database indicates that this fact has been cited in hundreds of news stories.
Some sources assert that it is the only earth-bound object observable from the moon.
As with many urban myths however, a moment's reflection reveals some problems. As with the Great Wall, it turns out to be too narrow to be discernible. To accomplish that trick, an object has to be long and wide. Competing claims for viewability have been advanced on behalf of a number of mankind's monuments, including the Great Pyramid, the Pentagon, and Kansai Airport in Osaka Bay. NASA helpfully explains that it's all a function of height, length and viewer distance.