Google reveals [almost] all:

http://en.wikipedia.org/wiki/Rangefinder_camera

Any further questions, get back to me! The article of course concentrates on photographic rangefinders of traditional optical/mechanical design (rotating silvered wedges). There are also large military rangefinders a couple of feet long and more, derivates of these for sportspeople, and all kinds of modern laser, infrared and ultrasound devices (for example, for measuring up rooms in houses).

Regards,

David