A coil of steel wire that became one of the best-selling toys of all time almost by accident.
Slinky (1945) was a genuine nationwide craze decades before "going viral" was a phrase — and it's still made and sold today. This is a genuine pre-internet product — a real, nationwide sales phenomenon decades before social media existed, spread by nothing but word of mouth, in-store demonstration, and traditional advertising. Many of these are still in production today, largely unchanged.
Naval engineer Richard James invented the Slinky after watching a torsion spring fall off a shelf and "walk." His wife Betty named it, and its 1945 department-store demo sold out almost immediately.
It's remained in production essentially continuously since, and still ships in close to its original packaging design.