Fiber Optic Lenses
A Fiber Optic Lens is great because they are flexible lenses that bend around corners, as well as when you need to look down a pipe or engine block, under a door, around a corner, or through a vent. Some of these have adapters you can use to hook up a camera or video camera to take pictures of what the lens sees.
No matter if you are law enforcement trying to look in on a situation where discretion is necessary, or just a mechanic or plumber who needs to see in tiny places, a fiber optic lens will help you get the job done.