I don't have the object here, but when going through a collection of donated camera equipment at the Photographic Historical Society, I came across a viewfinder for some camera (German?) that was meant to "project" a frame on your vision. It was meant to be used when keeping both eyes open. The rest of the guys in the group thought it was pretty neat. It didn't work for me because I do not really have binocular vision, I'm wall-eyed, and while each eye works pretty well, they don't work together.
That's the Voigtlander Kontur finder. I had one with my Voigtlander Vitessa-L, and I really liked it. You can see what's happening around the edges of the frame and adjust your composition accordingly. It's excellent for low light shooting, because you really have no light absorption from any kind of viewfinder system, but it presumes you have some other way of focusing.
It came in 35mm/6x9cm format and 6x6cm and could be used with the normal focal length on each respective format.