Leice produced a number of such meters (or rather Metrawatt produced them for Leica). The best-known models are the Leicameters MC and MR; the former has a selenium cell, the latter (more desirable) has a CdS cell that runs of a PX625 mercury cell.
It works on the M1 through M4. (I'm not sure if on the MP as well, but there it would be kind of pointless). It couples to the shutter speed dial by means of a pin which fits into the small cut out section in the speed dial. That way it is sort of half-coupled to the camera and relatively comfortable to use. (However, it adds bulk and people tend to scratch their camera top plates with them.) The metering area roughly corresponds to the 90mm frameline.
The main thing it has speaking for it is the shutter speed coupling, but if you can live without that, there are better meters out there IMHO; the Leicameters, even the MR, aren't particularly sensitive, tend to be a little fiddly in use and it's surprisingly difficult to find a working one, as many appear to be dead by now. There is a service guide somewhere on the net that covers taking the thing apart and cleaning it. The MR comes in two versions; the newer one (often called "MR4") has the metering button on top, where the original MR had it on the side where it was apparently prone to breaking off.
In a pinch, you can also take the MR off the camera and use it as a very small handheld meter with an awkward user interface. I had an MR4 for a while (but no Leica to go with it) and used it that way, hoping to reuse it if I ever bought an M[1234].
