A very unofficial fork of the Prolific PL2303 driver hosted at http://osx-pl2303.sourceforge.net/, compiled for modern versions of Mac OS X, including Snow Leopard.  Patches very welcome, particularly those that add vendor/device IDs.  Prebuilt PKG (just as good as a DMG!) available for download, works on Snow Leopard (Mac OS X 10.6) 32-bit and 64-bit kernels.