It is a widely held belief that the Great Wall Of China is the only man-made object visible from outer space, or at least one of two or three, depending on the claims of the Louisiana Superdome and Jebel Ali. But despite it being printed on the back of a Trivial Pursuit card, it's simply not true. The wall is thousands of miles long, but it's only about twenty feet wide, and not particularly contiguous.
While the Gemini V spacecraft was in space, astronauts Gordon Cooper and Charles Conrad made note of a number of visible objects, including a checkerboard pattern that had been laid out in Texas and the aircraft carrier that they were going to land on. Since the longest of aircraft carriers is just over 1100 feet, it stands to reason that a huge number of other objects would have been visible had Cooper and Conrad been on a sightseeing tour rather than a NASA mission. But perhaps they weren't far enough away.
The first problem comes in defining exactly where outer space is. Is it one hundred miles up? One thousand miles up? Halfway to the Moon? On the Moon itself? At the altitude the Space Shuttle orbits (typically between 190 and 350 miles above sea level), the Great Wall is a thin, barely visible line. But other objects are visible as well, such as the Golden Gate Bridge, the Panama Canal, and the Fresh Kills Landfill on Staten Island. The Eisenhower interstates are also faintly visible as well, as well as crisscrossing patches of farmland adjacent to the Great Wall itself.
What about the Moon? There are only a few eyewitness accounts, but none have reported seeing it. To the contrary, they've all clearly stated that it's not visible. Moonwalkers Neil Armstrong and James Irwin stated that it was not visible, and photos from the Apollo missions prove it as well. The continents themselves are hard to make out from the surface of the moon. A wall twenty feet wide doesn't have a prayer.
Aside from eyewitness accounts, one can calculate the possibility of seeing an object from the moon simply by knowing the distance between the Earth and the moon and the resolution of the human eye. With 20/20 vision, an object would need to be almost a half a mile in two dimensions for it to be visible from that far away. The entire Earth fits in less than two degrees of arc when viewed from the lunar surface. Not much room for distinguishing features.
So what's the last man-made thing you can see as you travel away from the Earth? Well, assuming it's not self-illuminating, like any number of cities whose light is visible at night, the objects that would be the last to be obscured would be the Dutch polders.