LaCie (the manufacturer probably best known for their external hard drives) provides a freeware Mac OS X backup application called...

SilverKeeper 1.1.4

SilverKeeper is a freeware application from LaCie that automates backups to external storage devices like hard drives, and other removable storage devices that have IDE, ATA/ATAPI, SCSI, USB, or FireWire interfaces.

I read about it on a LifeHacker Top 10 list. Note that while it is freeware, LaCie requires registering your name and email address.