Change my voice on the fly.

Is there any software which can change my voice on the fly as i talk into the microphone so users on the other end of MSN/Skype/ETC hear me as a chipmunk/darth vader etc.

I am not prepared to buy or try any software so it must be free.

Audacity doesn’t do anything in real time. If you need it done as a post production effect three days later, then the the Audacity pitch changer is for you.

Google is good.