The Slinky's inventor, Richard James, was initially working on a device that would keep equipment steady on ships when he discovered the coil's unusual movements. James's creation is an example of an "accidental" invention borne our of a completely different intent.
James first discovered the Slinky's use as a toy when he accidentally knocked over some prototypes and noticed how they moved unusually as they fell. Betty James, Richard's wife, helped him market the toys and even came up with the Slinky name. From these humble beginnings, Richard and Betty managed to successfully market the world-famous Slinky.