Dive Compasses, frequently not used, but extremely valuable when needed.
Alec provides tech tips about compass usage for divers. In the following video he demonstrates compass mounting options that provide for more efficient use of the compass.
A key fact that he points out is that if you use a compass more than once in 25 dives, it is a lot. However, such infrequent usage does not diminish the importance of having a compass readily available for the rare cases when you actually need it.
Watch Alec's presentation about compass usage in teh video below.