The following utilities provide either one-way or two-way synchronization of files and folders. Because they offer neither duplicates nor archives in the sense discussed in this book, I do not categorize them as true backup software.
AASync 1.2 (www.aasync.com; two versions: one free, one $19)
iBackup 5.0 (www.grapefruit.ch/iBackup/; free)
iMsafe 2.0 (http://homepage.mac.com/sweetcocoa/; $19)
iShelter 1.0 (www.brattoo.com/propaganda/; $10)
Phew 1.0 (www.substancesoftware.com; free)
Synchronize X Plus 3.0 (www.qdea.com; $30)
zsCompare 3.0 (www.zizasoft.com; $35)