Adapters to use M42 lens on other non-M42 cameras

There are two different factors which influence a possibility of existence of an adapter between M42 lens and non-M42 body.
First, the registration distance (the distance between the film plane and the top of the mount on camera body) dictates the possibility to make an adaptor without using an optical element.
Second, the width of a non-M42 mount should allow to mount the M42 lens on the body.
The registration distance of M42 mount is 45.5mm. So, for example, if one wants to use the M42 lens on Nikon body (registration distance = 46.5mm) and keep the correct infinity focus, the adaptor must have an optical element, because the registration distance of Nikon is larger than the registration distance of M42. On the other hand, the M42 adaptor to pentax K (registration distance = 45.5) does not need an optical element but the K mount needs to be wider than M42 so the lens and the adaptor together will fit. This is, fortunately, the case and M42 lens to K-body adaptors are readily available.
Note that there are several M42 lens to Nikon body adapters by different makers without the optical element. Such adapters can be used for close-focus work only, as the infinity focus is not correct.

Below is the list (ever growing!) of the M42 adaptors that I and other users know exits. Only adapters that allow for infinity focus are listed. Note that in most of the cases the M42 lens needs to be stopped down before taking picture as most of the adaptors do not provide automatic diaphragm option.