Little Alter Boy by soundtoys is a hugely powerful FREE plug-in centered around manipulating vocals, but it can also be used for other application for very interesting effects.
The plug-in itself is very simply laid out, with pitch, formant, drive and mix knobs as well as 3 different modes including transpose, quantize and robot (for a quick Daft Punk vibe). The pitch knob comes linked with the formant knob to quickly transpose any vocal by simply moving one knob up or down. The drive knob gives the plug-in brings a classic tube saturation model from decapitator.
Little Alter Boy comes with a range of presets to make the experience even more swift and satisfying. If you're a producer and like to have vocals in your songs, this free plug-in is one of the most powerful tools out there for manipulating and making a vocal perfect. A necessary tool in any producers arsenal.
You can find it for free here