The VC Meter II (Voigtlander Cosina) is a small reflected light meter that sits in your cold or hot shoe. It's quite compact, and looks good on a Leica and many other cameras. I have one that I switch between my IIIc and my M3.
You have to push the button and turn the aperture dial until the center LED lights up, then transfer the settings to your camera lens and speed dial. So it's like using a separate hand-held meter, except that it perches handily on top of your camera.