Asylum Studio Productions has released PAEIN, a free / donationware "Universal Panner" that allows fine tuning of pan law and other panning characteristics for both mono and stereo tracks.
Using various curve algorithms for both the pan law and channel attenuation along with a very detailed graphical representation of the pan settings, a user can very quickly relate what they see to what they hear. This is useful since most DAWs do not give you many options to control how the built in panners work.
The initial release is available for Mac OS X Universal Binary and supports AU and VST via the Pluggo runtime. RTAS and windows support is scheduled for later this year.