Free Mac Music Software: GWS RanPan is a free random pan effect for OS X, which can be synced 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. Now any audio application that hosts Audio Units can take advantage of this plugin to randomly pan sound around in sync with the beat!

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.

Universal GWS RanPan is a Universal Binary, includes a 64-bit binary, and is compatible with Mac OS X 10.5 Leopard and 10.6 Snow Leopard.