It's attached to a window and is found in Scandinavia. What is it? The answer is below the fold.
It's a skvallerspegel. That's Swedish for "gossip mirror". Bjork Brodern says these can be found on some older streetside houses. The mirrors are angled at 45°. People inside can glance (or stare) at the mirror to see what's happening out on the street.
You can buy them online if you'd like to add them to your home or workplace cubicle.
I have nothing but praise for any invention that improves situational awareness.
-via Wrath of Gnon
Here's an example of the mirror mounted on a house in Denmark.
That's the Sankt Jørgensbjerg neighborhood of Roskilde, which was a fishing village long before automobiles were even a concept. It's now a very desirable part of town. With Google Street View you can find a few other mirrors in that neighborhood.
Rear-view mirrors for your windows is fine, but I'd think more important on your door. Seems like it belongs in very high-crime countries, rather than Sweden, though not hitting pedestrians as you swing open your windows is good as well.