The Ricoh GV-3 External Viewfinder is designed for the Ricoh GR IIIx camera system. The GV-3 attaches to the camera body via the hot shoe and stores nicely in the included case when not in use. Highlights of the GV-3 include 40mm frame and 1:1 aspect ratio marks.
I like it for its compact size and it still offers a bright view. I think there are less expensive options out there, but the brightness of this finder and the perfect integration with the GRIIIx makes it worth the price.
I bought this thinking it would be helpful for composing with the camera but it isn't really that helpful. The center of the viewfinder doesn't match up with the center of what the lens sees, so you have to learn to compensate when looking through the viewfinder. There are no indicators to show you what shutter speed/aperture are set (which I had read), but I also realized that you cannot see where the camera is focused either.