The Dekatron is a special vacuum tube which serves as an empty state decade counter. The tube was often used as a prescaler to other electronic or mechanical counting devices back in the 1950s. Dekatrons could operate at a fairly high speed (considering the speed of most logic devices in their era), counting at up to 100 kilohertz. A Dekatron has a number of cathodes arranged radially, forming 30 cold cathode diodes, mounted on a common anode disc. An input pulse arriving in sequence at each of three inputs, controlled by an external phasing network, would advance the discharge forward by three cathodes. Every third cathode is connected to an output, allowing the device to be used as a decade counter.
The gas at the cathodes is ionized in a rotating sequence, stepping in a direction controlled by the phasing network. This process is very fast, making the Dekatron a good counter or prescaler for use in Geiger counters and other high-speed aplications.
Now, here's the fun part: Each of those cold-cathode diodes is visible, and the tube is filled with neon gas. Thus, each diode glows bright orange as it is discharging! If you were to watch a Dekatron in operation, you would see points of light spinning circles around the top of the tube. Thus, the Dekatron is usable not only as a counter, but also as a display.
I really wish there were a good way to attach a picture of a Dekatron in operation to this writeup, but, since there isn't, go see one spinning away at http://electricstuff.co.uk/count.html