I have a TLA140 (Kyocera CONTAX) and a FC-E (MINOX). Both are not that powerful, but they are extremely small, so I bring one of them almost anytime even when I don't have any particular plan to use it.
As John Roberts mentioned above, TLA140 is a very small and light flash which works with a CR123A lithium battery. I love this small thin silver-finished flash, it really matches my old chrome Nikkormat. The FC-E, which is originally designed for a dedicated flash for the Minox 35 series, is almost the same size (though a little bit thicker) as the TLA140. It works with only two 4AA batteries, GN18 which is tad powerful than the TLA140(GN14), and it has two auto setting mode (Full/F4/F5.6/ISO100). The main drawback of the FC-E is that it can't be mounted on all camaeras without an adapter. The shoe part of the FC-E is located at the hollowed area of the flash body, so it can't be mounted directly on the cameras which have a flat upper surface you find with most of the rangefinder cameras.
TLA140 - Contax (Kyocera) / GN14(m) / Full, TTL, test / w56mm x H74mm x D21mm
FC-E - MINOX / GN18(m) / Full, 2auto, test / w74mm x H55mm x D30mm / High trigger voltage (over 200V)