I used to use names of mythical creatures, but I forgot all the myths while I was at Uni, so since then, I name them after black things. In recent memory I've had box, magic, art, mail, out, guard, ball and most recently death. The previous firm I worked for used names of Scotch whiskies, but they complained when I called a machine bunnahabhain (I wonder why?). The one before that used the names of rivers or streams in the local area, alphabetically (annan, bladnoch, closs, and so on). Another used names of hills. Uni used names of villages.