This is an email inbox counter, using a single IN-16 nixie tube.
I originally thought of making this wireless, but on second thought, I'll made it USB, as it seems the sort of thing you'd use in the vicinity of a computer. USB also serves as a handy power source, as I found a DC converter, designed for nixie tubes, that takes low voltage input.
I intend to pack this in a small mint tin, so space efficiency is a major consideration here. I started out thinking discrete NPNs to drive the digits, but instead, I used a dedicated nixie driver IC to save space. Not only does the IC take less space than the transistors would, but it also reduces the pin count required from the driving logic. So I could use a 20-pin ATtiny2313 instead of the 28-pin ATmega8 that I started with.
I got a DC converter module for an eminently reasonable $10. But it still takes up valuable real estate and might be redundant. Why not use the AVR to drive a PWM boost converter? My only concern is that 5V to 170V seems like a pretty big boost, but others say it can be done.
I'll probably do up the first breadboard prototype with the DC module and then look into the PWM thing for the final circuit. It would be handy to have the module around for rapid prototyping anyway.
In a single-tube design, one can reduce the required circuit board real estate by stacking the nixie tube directly over the driver chip. That is hard to do if you directly map the chip output pins to the tube inputs, so the trick is to rotate the tube to the more physically convenient position (anode wire orthogonal to chip) and remap the inputs in software.
This nixie tube counter displays the count of my gmail inbox. For a sense of scale, the counter is mounted on top of a 22 inch computer monitor, attached by a velcro strip.
The top of the case is a little scratched from the build process, so I'll make another one soon (I have about ~100 such cases).
The module on the left is a $2 PL2303 USB serial adapter from ebay, which supplies both 5V power and instructions from the computer. The module on the right is the high-voltage anode driver for the nixie. The chip in front is an Atmel AVR attiny2313. The nixie tube is a Soviet IN-16, brand new from old stock. The cathode driver for the nixie is a Soviet K155ID1 (74141 clone) and is mounted underneath the tube.