Olympus made the VF-1 optical viewfinder to match its 17mm lens when it released the EP-1.
Apart from that, I believe all other accessory viewfinders are actually made for the rangefinder market, and are all designed to match particular focal lengths on 35mm film; you'll have to adjust for the crop factors of EVIL cameras (2.0 for µ4/3, 1.5 for the newer Sony and Samsung):
- Leica (Leica gear, Leica prices)
- Zeiss (pricy, but contrasty and well-corrected)
- Cosina/Voigtländer (less pricy, still rather good)
- Russian. Fedka is where I get Russian gear, and eBay's always a good choice.
There are none that will zoom automatically.
There is one that I know of that can be zoomed: the Voigtländer 15-35mm finder, which is not particularly cheap. It has two models, one provides adjusted scales for 1.3x and 1.5x crop, the other for 1.3x and 2.0x crop (1.3x is the crop factor of the Leica M8, which should also help explain the price point). The Russian 'turret' finders and the Leica Universal Wide Angle Finder (which is expensive) can be adjusted to particular focal lengths (e.g., 18, 21, 24), but not smoothly between steps like the CV.
Of note is parallax correction, but it's actually slightly good news: EVIL cameras have a smaller lens/shoe distance, and so parallax will be smaller than the film (or film-sized) cameras these finders were designed for. Many finders include parallax correction marks (only telephoto finders tend to have dial-in correction), but the best practice is always to be a little generous in your framing.
Regarding focal lengths, something to remember is that you can be "close enough". There is a certain amount of built-in error with almost any viewfinder, and accessory finders more than most. E.g., a 35mm finder would work perfectly well with the Olympus 17mm lens, and adequately with the Panasonic 20mm lens (though there is a CV 40mm finder).
All that said, I've used the electronic viewfinders for the EP-2 and GF1. The GF1 was lousy; worse than guessing. But the EP2 finder was actually quite good; definitely worth seeing for yourself.