Now more accurately marketed as "Spy Eye: Lets you see around corners", this little gadget was, for many years, known as the Seebackroscope. It doesn't really let you see behind you, but does let you see back over your shoulder or around a corner, sort of. A great little novelty toy is finally available again!
 
Manufacturer's safety information: Not suitable for children under 3 years. Choking hazard due to small parts.