mac plugin ideas?

hi

Being primarily a pc user I'd love a few mac free plug in suggestions so that I can assist someone using AM on mac.

We want to process live voice to achieve a melodic output i.e. voice becomes melody.

Some possible approaches:
- the voice could trigger a sound file or loop. A sample player of some kind?
- the voice could trigger a synth.
- a follower follows the voice with a replacement sound e.g. sine tone

Any suggestions much appreciated!

brendon