GSnap - With GSnap you can get an auto-tune effect.
ZynAddSubFX - ZynAddSubFX is an open source software synthesizer for Linux, Mac OS X and Microsoft Windows.
Revoice Pro - Revoice Pro is an audio plugin used for vocal tuning, time correction and ADR.
Qsynth - Qsynth is a simple Qt4 application wrapper for the ...
Voxal - Modify your voice in any application that uses a microphone. From girl to alien, the voice options for online video games, avatars and podcasts are limitless.
Surge XT - Open-source subtractive-hybrid synthesizer formerly sold commercially as Vember Audio Surge.