TenByTen Software's GWS RanPan is a free randomized panning effect for OS X that can be linked to tempo.
Here's what TenByTen has to say about it:
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. Now any audio application that hosts Audio Units can take advantage of this plugin to randomly pan sound around in sync with the beat!
Universal GWS RanPan is a Universal Binary, including a 64-bit binary, and is compatible with Mac OS X 10.5 Leopard and 10.6 Snow Leopard.