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
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.