Velcro
Swiss electrical engineer George de Mestral invented his first touch fastener when, in 1941, he went for a walk in the woods and wondered why Burdock seeds clung to his coat and dog. He discovered it could be turned into something useful. He patented it in 1955 and subsequently refined and developed its practical manufacture until its commercial introduction in the late 1950s.
Centrifugal clutch
Thomas Fogarty, who is also credited with inventing the balloon catheter, is credited with inventing a centrifugal clutch in the 1940s although automobiles were already being manufactured with centrifugal clutches as early as 1936. There is also a design for an 'automatic clutch' (essentially the same device) in the 'Meccano Magazine' of June 1931. But centrifugal clutches were used in railway locomotives much earlier (before 1858), and referred to (in relation to electric motors) in a patent of 1899. 
A device such as this was reportedly used in the Taylor Aerocar roadable airplane of the late 1940s and early 1950s.