tewe 35-200mm viewfinder
This is a high-quality (West) German-made zooming viewfinder that covers
the range 35-200mm natively, and, with a supplied attachment mini "wideangle lens" that fits
over the front of the finder, extends the view down to 28mm.

Parallax compensation is achieved by moving a lever under
the finder which is marked off at 3, 6, 15 feet and infinity

The image through the finder is quite nice, but mine is a little dimmer
from age - it could use a thorough cleaning. The foot is very wide - too wide
for my (Soviet) Russian rangefinders. However, the foot has a slot cut into it
along the whole length, leaving only a couple of short bits of metal
at either end to slice through with a hack saw (or the craftsman's equivalent precision tool thereof) to make it narrower (see
picture below).

This cut-away "slot" was evidently provided for the reason
that some accessory shoes were narrower gauge than others in the 50's.
I'm a little loathe to do this to my prestine viewfinder just for the sake of mounting it on a $50 Soviet rangefinder, though! I have found that it fits any modern shoe without trimming, which leads me to this next bit...
As a Powershot G9 Optical Viewfinder Alternative?
The Canon Powershot G9 high-end digital compact camera has a "35mm equivalent" zoom range of 35-210mm - as it happens, the same as this zooming viewfinder, save for the final 10mm of zoom. Many users of the G9 complain that the built-in optical viewfinder in the camera is a plastic abomination - murky, badly parallax corrected (if at all) and barely usable. Of course, the G9 has a glorious 3" LCD, but it's not always convenient to use that, and some people downright dislike framing shots that way. So why not slip something a little more comfortable onto your G9 by sliding one of these Tewe viewfinders into the hotshoe? I know that's what I'll be doing when I buy the G9 soon!
Stay tuned for further reports on this combination. I should point out that there do exist other zooming viewfinders of this vintage that cover this focal range, so do not despair if you can't track down a Tewe specifically for this purpose - you might find a Leitz/Leica one or similar. Oh, and before anyone asks... mine is not for sale.
Other Gear: FED-2(f) Zorki-1(d) KMZ Turret Finder