That would be the bicycle.
Some rather breathless thoughts on the social implications of this technology from a contemporary Atlantic Monthly article by W J McGee:
A typical American device is the bicycle. Invented in France, it long remained a toy or a vain luxury. Redevised in this country, it inspired inventors and captivated manufacturers, and native genius made it a practical machine for the multitude…Typical, too, is the bicycle in its effect on national character. It first aroused invention, next stimulated commerce, and then developed individuality, judgment, and prompt decision on the part of its users more rapidly and completely than any other device; for although association with machines of any kind (absolutely straightforward and honest as they are all) develops character, the bicycle is the easy leader of other machines in shaping the mind of its rider, and transforming itself and its rider into a single thing. Better than other results is this: that the bicycle has broken the barrier of pernicious differential between the sexes and rent the bonds of fashion, and is daily impressing Spartan strength and grace, and more than Spartan intelligence, on the mothers of coming generations. So, weighed by its effect on body and mind as well as on material progress, this device must be classed as one of the world’s great inventions.
Source: From the American System to Mass Production, 1800-1932, by David Hounshell.