http://www.hamrick.com/
This guy makes VueScan, well worth it in my opinion. It supports hundreds of different models of scanners and is updated once every couple weeks to include more scanners.
I guess you could say it's the GraphicConverter of scan software.
The great thing is that it supports a lot of older, but highend SCSI scanners that don't "work" under OS X. We bought it at work to use one of our $9,000 Linotype-Hell scanners that "isn't supported" under OS X.
It often has a lot more options and configurability than the basic software that's shipped with the scanner.