rsync hfsmode is a patch for rsync to enable recognition of Mac OS X HFS resource forks and Finder metadata, and to copy them to a remote filesystem. The destination system can be any OS and filesystem that supports rsync, so you can use rsync to archive Mac OS X files to servers running Linux, Solaris, et cetera.
Unison runs on both Windows and many flavors of Unix (Solaris, Linux, OS X, etc.) systems. Moreover, Unison works across platforms, allowing you to synchronize a Windows laptop with a Unix server, for example.
rdiff-backup backs up one directory to another, possibly over a network. The target directory ends up a copy of the source directory, but extra reverse diffs are stored in a special subdirectory of that target directory, so you can still recover files los