BMW Z1
The Z1 was a strange little BMW sports car with removable doors and a style that was unlike anything else on the road. Although the Z1 looked like a concept car, it was indeed a production model. The short wheelbase allowed the Z1 to perform like a true sports car on tight roads with lots of twists and turns (via Hot Cars).
The Z1 was a very limited production roadster produced from 1989 to 1991. That limited production run meant that the Z1 has steadily increased in value over the years. The fact that it was a roadster that had removable doors made it an even more unique proposition than just about anything else on the road.