Modifying your computer's case by removing the bland beige paint and painting it black with flames (or whatever you want!) adding extra fans and windows, cleaning up your inside wiring with plastic tubing, and then watercooling your CPU or videocard. Started out with Lan gaming fans who wanting to trick out thier rigs but its now spreading to the mainstream.

Modifying a computer to make it look cool. Generally followed by the realization that computers don't make you cool. Plastic windows so you can see your electronics, neon lights, and loud fans.