I asked on Twitter if there was a voice changing software for the Mac and @Mosaeus came through with the suggestion of Voice Candy.  It was offered as a part of the MacHeist package (I have no idea what the heck MacHeist is, I don't know but remember seeing Chris Pirillo mention it) so I Googled and found the developer's site.

Please enjoy the demo I recorded of the different sounds including: Mouse, Bad Sci-fi, Radio, Robot, Vader and Chipmunk!  I couldn't stop giggling once I hit "play".