TWAIN Drivers on Mac
Hamrick VueScan

This is a "Swiss-Army-knife" scanning application that supports hundreds and hundreds of makes and models of scanners, even many obsolete ones which don't have direct Mac OS X support. 

Download and evaluate the trial version. 

I use it with my Brother MFC-6490CW, which does have Mac OS X drivers, but I'd rather use VueScan than use the Brother or Apple scanning software. Although VueScan has TWAIN support, I use it as a stand-alone scanning solution to output PDF, TIFF and JPEG.