What kind of transformations do you want? Almost any audio processing (filtering, waveshaping etc.) will transform a voice...
For the far-out, things like ring modulation (think Daleks) are a perennial favourite, and of course delay effects like comb filtering and flanging. Some FFT processes work well on voices too. And then there's vocoding!
But for the semi-realistic, the best are probably speed/pitch and formant effects. Have a look at Tim Rozendal's recently posted DiracLE object - it does nice formant munging that will subtly - or not-so subtly - change a voice,
@Roger - Yeah I have tried all sorts of filters and the standard synthesis effects but I was wondering if there are some more specific methods that I dont know about.
Ill take a look at the DiracLE object - havent heard about it before. Thanks :)
@Grizzle - thanks Ill take a look at this when I get home :)