Argus C3, from Wikipedia
Leica are usually held up as being the big drivers behind the spread of 35mm. I've always doubted that claim, given that they've always made fairly niche/luxury (read: expensive) products. They certainly raised the profile, given the sheer range of high-profile users.
Well, doing some web trawling on older cameras I came across the Argus C3 that lays a fair claim to popularising the 35mm format in that it actually got it into the hands of the mass population, at least in the US. I love the quote
The profusion of knobs, gears, buttons, levers, and dials on the camera lent it a "scientific" look that was found in customer surveys to be one of the things buyers most liked about the camera.