Tokyo Toilet Project has installed transparent restrooms in two parks in Japan. Each park received a women's toilet, a men's toilet, and a multi-use toilet.
Their see-through walls serve two purposes: They allow people to gauge their cleanliness without entering them and you can more easily tell when someone is using them. They also glow in colored hues, creating for a pleasant visual display at night.
When the restroom is not in use, the walls are see-through, but when the door is locked, the walls turn opaque, blocking the user from view. However, make sure you remember to lock that door or you'll be on display for everyone outside!