The fun (and beautiful) mid 1950's Argus C3 Rangefinder Camera:

Often called "the brick" for it's size and weight, something like a million-billion were made and they are easy to come by.

The focus knob (front face, upper left in picture) drives a gear that drives the lens focus - KEWL!
A rangefinder works by splitting an image into two (stereo vision essentially).
The Argus C3 uses a window within the round focus knob and the left window above the lens (as seen in the picure above).
A rangefinder splits the image in two, and once the two are alligned, they are focused. Next you look through the far viewfinder for the "whole" picture and shoot!
1950's Argus 35mm range finder