A dental mirror is a very highly useful tool, used by dentists, mechanics, and computer repair-persons.
It is highly useful for looking down inside any oddly shaped space, such as somebody's mouth, between cards inside a computer, down various ducts, tubes, and around behind stuff.
The dental mirror consists of a flat mirror, about one inch in diameter, on a long handle. At its end, there is a short offset piece with ball joints where it connects to both the mirror and the handle. This allows it to be positioned in various different angles. I think there are also dental mirrors which have a fixed 45 degree angle at the end; these may or may not be less useful. The mirrors used by dentists probably have a fixed head, and are disposable.
There is also a larger version available, which is an inspection mirror at the end of a telescoping handle. This version is undoubtedly less suitable for dental work, as it telescopes to two feet or more in length, however, it would be the perfect thing for looking at a car's underpinnings without getting it on a jack.