Another +1 for the Olympus VF-1, works well with focal lengths between 35 and 40mm, although its frame lines are a bit tighter than 35mm. As jimmy2thou mentioned above, the VF-1 works well with the Lumix 20mm lens. Mind though that the VF-1's framelines have 4:3 aspect ratio, as it was designed for the Micro Four Thirds system. The top and bottom lines may correspond rather well to an actual 35mm 3:2 field-of-view, though.
I can't recommend the Helios finder, as the framelines aren't very accurate, politely speaking. Impolitely speaking, it's unusable bull****. I had it for my E-P1, and absolutely didn't like it. Another problem with it is that it won't sit tightly in the flash hot-shoe, at least my copy didn't.
The Voigtländer 35 and 40mm finders are reputedly good.