A nickelodeon or 'coin-piano' is any automatic instrument that is coin operated; a coin in a 'slot' (usually a 'nickel') plays one song on a roll. These instruments range from a vast variety of almost all American makers such as Seeburg, Cremona, Coinola, Wurlitzer, and Nelson-Wiggen, these just being a small handful of the once many manufacturers of these.
Most nickelodeons are based around a piano with a few exceptions. The Encore Automatic Banjo, for example is just a self-playing banjo! The Mills Violano Virtuoso is an instrument with a real, self-playing violin accompanied by a piano. The Mills firm made this instrument further unique by making the action electrical rather than pneumatic.
As for rolls, nickelodeons are unique in that many manufacturers did not have their own scale for machines but instead based it around 3 universal scales, the A, the G (Or 4X), and O roll. Each of these has their own advantages as well as limitations. The A roll is limited to only piano and 2 other instruments; the G roll has more instruments as well as percussion but with a smaller piano scale, and the O roll has a 'solo' instrument.
There are exceptions though. Seeburg's largest instrument, the style 'H' (pictured above) plays its own roll; all of Wurlitzer's nickelodeons play a their own scale with rolls issued by Wurlitzer themselves. The Link Company manufactured nickelodeons with an 'endless loop' roll system, also playing their own rolls, and the same goes for Mills, North Tonawanda, and other early machines.
Many of these scales also used existing player piano arrangements. Percussion, registers, as well as some other modifications to make these work for these were made. 'Pianola-ists' such as J. Lawrence Cook and Pete Wendling had their arrangements expanded by roll companies for these instruments, and by doing so easily created a massive library of rolls with arrangements still being very good.
A Link Roll System is Fascinating to watch, and even more so to hear the music!
The inside of a Mills Violano Virtuoso
In terms of popularity, nickelodeons are immensely common. There are thousands of small 'A' roll pianos around, called 'cabinet' pianos due to being compact and not having a visible keyboard. They survived well during the prohibition (which killed the market for many larger instruments) due to being perfect for the small speakeasy.
You probably won't find many nickelodeons in a bar today due to the prohibition and decades after killing most 'old-time' saloons and speakeasies, but they have survived well in collections, museums, as well as being popular at some amusement parks. Disneyland and Disney World, as well as Knotts Berry farm had dozens of working nickelodeons on display up until the 1990's.
Nickelodeons are a 'must have' for collectors, and smaller instruments are almost always priced in an affordable range for the hobbyist.