I got really impressed by the visual LED swinging-light MIDI metronome / tempo indicator/meter unit that Björk is using on her Volta tour (and possibly other tours). It consists of a red light that blinks on every beat, and a set of lights on each side. The light swings left to right and back, precisely with the master tempo. I saw it there, and I can't take it out of my head.
edit: Apparently it's this unit.