ZynAddSubFX - ZynAddSubFX is an open source software synthesizer for Linux, Mac OS X and Microsoft Windows.
GSnap - With GSnap you can get an auto-tune effect.
Qsynth - Qsynth is a simple Qt4 application wrapper for the ...
Revoice Pro - Revoice Pro is an audio plugin used for vocal tuning, time correction and ADR.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.
CrispyTuner - Fully featured vocal tuning suite.