This is a random pan effect, which can be linked to tempo. I couldn't find an effect that did this when I wanted to apply it to my own music project, so I whipped it up myself.
Version 2.8 adds 64-bit to the Universal Binary, and fixes a problem with the number formatting in the text fields of the user interface. This version requires Mac OS X 10.5 or greater. For more information, see "GWS RanPan Read Me" in the zipped disk image file.