Revoice Pro - Revoice Pro is an audio plugin used for vocal tuning, time correction and ADR.
UVI Falcon - Sampler/synth hybrid instrument
GSnap - With GSnap you can get an auto-tune effect.
Pianoteq - Pianoteq is a virtual piano that runs on your computer.
LinuxSampler - LinuxSampler - free, open source pure software audio sampler with professional grade features.
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.