You're not a real geek until you've built a Nixie tube clock.

In the 50's and 60's, these Nixie vacuum tubes served as the earliest digital display for electronic instrumentation, and are enjoying a popular revival among hobbyists for their warm, neon-like glow and retro esthetic. 

Nixie IN-8 four digit clock

Nixie 4 digit

Nixie IN-12 Chess Clock

Nixie chess clock

Fluke Voltmeter Nixie Clock

Fluke Nixie clock
When new, this 60's-era digital voltmeter cost upwards of $1,000. Although the voltmeter circuitry no longer functions, the tubes are beautiful and it makes a cool clock.