Flutterbird is a free, open-source effect plugin for adding pitch and volume fluctuation to incoming audio. It can be used for traditional wow/flutter effects as well as more chaotic, extreme modulation. Flutterbird has four separate oscillators with adjustable speeds that can be mapped to either pitch or volume.
PitchPerfect is a free guitar tuner that eliminates the need of tuning a guitar conventionally and will automatically detect the note you are playing. Tune your strings quickly and easily to any note you want! No longer are you constrained by standard tunings, PitchPerfect includes more than 50 alternative tunings.
SoundTouch is an open-source audio processing library for changing the Tempo, Pitch and Playback Rates of audio streams or audio files. The library additionally supports estimating stable beats-per-minute rates for audio tracks.
Tempo (time stretch): Changes the sound to play at faster or slower tempo than originally without affecting the sound pitch.
Pitch (key) : Changes the sound pitch or key while keeping the original tempo (speed).
Playback Rate : Changes both tempo and pitch together as if a vinyl disc was played at different RPM rate.
The SoundTouch library is intended for application developers writing sound processing tools that require tempo/pitch control functionality, or just for playing around with the sound effects.
The SoundTouch library source kit includes also an example utility SoundStretch for processing .wav audio files from command-line interface.
Access the source codes in gitlab or download the compiled executables.
While working at the music and neuroimaging lab at Beth Israel/Harvard Medical School in Boston, I developed a quick online way to screen for the tonedeafness. It actually turned out to be a pretty good test to check for overall pitch perception ability. The test is purposefully made very hard, so excellent musicians rarely score above 80% correct. Give it a try!