This is [Michael Ossmann's] RGB LED stroboscopic guitar tuner. If his name is familiar that’s because we mentioned he’d be giving a talk with [Travis Goodspeed] at ToorCon. But he went to DefCon as well and spent the weekend in his hotel room trying to win the badge hacking contest.
Despite adversity he did get his tuner working. It’s built into a toy guitar that he takes on road trips with him. By adding a row of RGB LEDs between two of the frets he can use the vibration frequency of an in-tune string to flash the three different colors. If the string is not in tune the three colors will dance around but matching it with the LED frequency produces a stable color. He then uses that big yellow button to advance to the next string. See his demonstration after the break.
This is basically a built-in plectrum tuner that uses one LED package instead of two.