1990 Mazda Miata
This car single-handedly revived the roadster. Maybe the biggest "Why didn't I think of that?" event among automobile manufacturers in the early '90s. When introduced, it had no direct competition (the MR2 was closest in terms of basic specs and sportiness, but wasn't a true convertible). At the time of its introduction, "roadster" meant an aging British or Italian convertible that did little more than leak in people's driveways.
Okay, I've had enough. Somebody else can cover the Model T, the '32 Ford coupe, the Willys Jeep, the Toyota Prius/Honda Insight (whichever came first... I forget), the Volkswagen Beetle, the first muscle car (Dodge Charger? Chevelle? Nova? tempest GTO? Whatever...) and a bunch of others that shouldn't be hard to single out as significantly new ideas.