KiSS originated in Japan, and its name comes from the phrase kisekae ningyou, which loosely translates to "clothes changing game". It was originally developed in 1991 for a NEC computer, but since then the specification has been formalised (and broadened, to allow for more capable hardware). This is known as the KiSS General Specification (KiSS/GS), and is available on The Big KiSS Page, maintained by Dov Sherman.—Tigger's Little KiSS Page
Back in 1999, when I started surfing, KiSS was new (outside Japan, that is; it had been developed in 1991, and the first specification dates from 1994) and exciting. In 2003, KiSS dolls were still widely exchanged, but it seems that the format is not developed any more. And that is a pity, because it showed great promise.
KiSS is short for Kisekae Set System, it was designed as a nostalgy thing, to re-create paper dolls, a popular Japanese toy, on the computer. What makes it interesting beyond this goal is that it represents something sorely missing in the enthusiasts' game creation world: a clearly defined data format.
It does not matter whether the individual KiSS viewers are open source or closed source, free or proprietary or whatever. The important thing is that the KiSS format itself is clearly defined, completely accessible and completely non-proprietary. Consequently KiSS viewers, just like emulators, exist for most computer systems. Anyone can create KiSS dolls, anyone can write a KiSS viewer.
KiSS dolls consist of several cells and are usually distributed in LZH archives, the archive format predominant in Japan. Most KiSS viewers can read them directly out of the archives, just as most emulators do. Some give you the option to associate them with LZH archives, but that might not be such a hot idea if you plan to ever download something else from a Japanese website, too.
