A simple, free, open-source application for singers who want to find pitches on their phone they way they would with a pitch pipe.
The interface is plain but straightforward, similar to a keyboard layout. Display can be either sharps or flats. The user can select from a variety of tones (sine, square, PWM, &c) and envelopes (organ, clavier, or fade-in/out).
Source code may be found here: https://github.com/mlzarathustra/tones