Partially coated (silvered) mirrors could do the job, if you've got some radiation of high intensity. For example, a laser. The intense beam comes out through the partial-mirror after a series of constructive reflections. Wiki doesn't have a good article on one-way windows. But, Googling it can throw a lot of results. These one-way mirrors are imaginary.
Say you're in a car and I'm out. Well, most of the cars today have these semi-reflective mirrors. If I try to peek into the car from quite a distance, I can't see you. But if I come closer enough (or stick to the glass), I can see who's hiding inside. But, this shouldn't happen with a perfect one-way mirror. I shouldn't see you at all. But, you can see me completely. Hence, we conclude that this is not practically allowable as it would violate second law of thermodynamics.
(Perhaps, there is a hiding answer in - Why does a window become a mirror at night?)