This is a clock that I built in 2001 using parts from an old calculator. The clock uses an antique digital display technology known as NIXIE tubes. It's housed in a peanut butter jar for the time being—I hope to find the time build a nicer enclosure soon, perhaps out of anodized aluminum.
The clock was built using perfboard. Despite looking more like a prototype than a finished piece, it has worked reliably for several years.
The display tubes are filled with neon gas (some argon too), and they glow when you apply 170v between of one of the number-shaped cathodes and the anode mesh. You can see both the cathodes and the anodes in the following picture (click to enlarge):
Because the driver ICs for these tubes are hard to get, discrete transistors are required to switch on and off the elements of the display—one for each tube digit and one for the flashing seconds lamp (28 all told).
The power supply and control ICs are on their own perfboard soldered behind the driver board.
Setting the clock is accomplished by holding a magnet up to the side of the jar. There are two reed switches inside that advance the minutes either fast or slow. (The switches also came from the calculator.)
Come to think of it, it's pretty remarkable that this kludgey mess works at all...
Full schematics, explanations of the circuit, and even a ready-made PCB can be found at Mike's Electric Stuff.