the Pentax K monut was adopted by a number of other companies.
Chinon, Cosina, Ricoh, and Vivitar are probably the best-known regular camera brands to use K-mount, aside from Pentax. Some others, such as Exacta, Quantary, Sears, and Zenit, also used K-mount for at least some models. Check
this site for a list of cameras that used K-mount.
Note that not all cameras from (AFAIK) any manufacturer used K-mount. Earlier Pentaxes and Zenits, for instance, used M42 (screw-mount) lenses. Some manufacturers, such as Vivitar and Sigma, made (and make) lenses for a wide variety of mounts. As a general rule, the "big names" (Minolta, Canon, Nikon, Pentax, Olympus, and a few others) made/make lenses only for their own mounts, but most or all of these have changed their mounts at least once. The bottom line: When buying a lens, verify what mount it uses; you can't tell just from the lens's brand. If the seller doesn't advertise the mount, steer clear unless you can positively identify the mount from photos -- and even that might be risky.
One more point: There are several variants of the K-mount. These add features such as camera control of aperture and auto-focus. For the most part, lenses with one K-mount variant can be used on all K-mount bodies, but you may lose some features. There are also occasional potential glitches; for instance, Ricoh's aperture-control system includes pins that get caught in some K-mount bodies. This specific problem can be overcome with a little celophane tape, but you could damage the lens mount if you're not aware of the issue and therefore don't take the necessary precaution.