Price: $160 (kit), $399 (fully built)
Joe Croft sent in his NixieNeon clock. What I really like about this kit is simply the number of neon bulbs and the use of less-common green neon bulbs as well. The manual itself is worth reading through. Clearly lots of work went in to designing this device. (Apparently the original intent was not to use a single processor!)
The software itself is open source under the GPL license. The core of the clock is based on an Arduino bootloader and library.
As far as the description of the operation goes, I won't try to paraphrase his description from the site:
The NixieNeon is a nixie tube clock designed with ring counters built using neon bulbs to allow the viewer to see the electronics perform the actual counting of time. This electronics kit offers the builder a glimpse back in time before semiconductors became inexpensive and plentiful. The Hour of the day is displayed using the twelve neon bulbs (the hour ring) circling the two 2 nixie tube digits which represent the minutes. The 4 green neon bulbs are for showing the positions for 12, 3, 6 and nine o'clock. They also blink to give a pendulum effect while the clock is operating.