Atomsplitter Audio has released Forma-8, a free formant filter that can morph between vowels using 4 corners of a XY pad - this allows you to make a range of human sounding effects on whatever you are applying it too. There is a modulation knob which allows you to mix in the amount used - this can make a shaking gated effect as it modulates the internal filters. There are also resonance and mix controls which allow you to apply the effect less powerfully and have just a bit of the effect, if you wish. As well as the vowel selection you have a choice of spoken and sung types and male and female genders.

Forma-8 is free and is available for Windows as a VST effect plug-in.