The Great Wall of China from space.NASA
First of all, it's worth noting that while man-made structures like the Great Wall are visible from satellites orbiting Earth, they're not visible at all from, say, the moon.
"The only thing you can see from the moon is a beautiful sphere, mostly white, some blue and patches of yellow, and every once in a while some green vegetation," Apollo 12 astronaut Alan Bean told NASA. "No man-made object is visible at this scale."
Secondly, the structures you can see largely depend on the weather and how far from our planet you actually are. In 2003, a Chinese astronaut even admitted he couldn't see the Great Wall from space, because the weather conditions weren't right.
Given the proper conditions, astronauts in the International Space Station can see major cities, the Great Pyramids of Giza, and some big bridges from space.