Guitar Tuner is an Android application designed by Peter Deelstra for guitar players who want to tune their instruments with maximum accuracy. The app provides a simple interface to visually check whether the pitch for each string is low or high, and lets you "see" the pitch change as you wind/unwind the string tuning machines.

One of the best features of Guitar Tuner is how it shows exactly how many cents (1/100ths of a semitone) a note is out-of-tune. This is extremely accurate, within 1 cent (1/100th of a semitone) of each note with sub-second response time. It also offers a wide range of alternate tunings, including regular tunings such as Minor Third, special tunings such Drop D, open tunings such Open C, lower tunings, and higher tunings.

Overall, Guitar Tuner is a great tool for guitar players of all levels. With its ease of use, accuracy, and range of features, it is a must-have for any musician who wants to keep their guitar in tune.