Look, that BOSS TU-2 has served you (and eleventy billion other guitarists
) well, but isn't it time for something a bit different in the pedal board mix? Stimmmopped is that very thing, which is a darling little contraption that uses LEDs rather than a microphone in order to tell you if your strings are tuned as they should be. Put simply, the device illuminates a string with a pair of lights, both of which are flashing at the frequency that the string should be vibrating at if it's in tune; if you're off, the illuminated part of the string will appear to be moving (thanks, stroboscopic effect!), and if you're on the money, the lights will appear fixed. Have a peek at the source link for more, but don't go building one yourself without a steady hand and a few vacation days to spare.