Flash and R-D1
Here's another vote for the Leica sf-20, now quite cheap and more versatile than I'd imagined until I began experimenting. For example, with the flat-folding little diffuser from LeicaGoodies (the Sfill, I believe it's called), you get nice effects with practically no extra weight (and the flash itself weighs only ounces).
Besides using this on my film rf cameras, I have also just tried it with the Lumix G1 and it works like a charm without damage to the camera. My best results thus far come when I the flash to A, match the aperture of the flash to that of the camera, and then jack up the flash's iso setting by quite a bit -- eg., G1 @ iso 200 vs. sf-20 iso @ 800. Not sure how consistent this will be under all conditions, but I will keep tinkering if necessary. .
Dan