If you want to mirror linux or unix files or directories to a remote server you should use rsync, http://rsync.samba.org. BackupPC uses rsync as a transport mechanism; if you are already an rsync user you can think of BackupPC as adding efficient storage (compression and pooling) and a convenient user interface to rsync.

Two popular open source packages that do tape backup are Amanda (http://www.amanda.org) and Bacula (http://www.bacula.org). Amanda can also backup WinXX machines to tape using samba. These packages can be used as back ends to BackupPC to backup the BackupPC server data to tape.

Various programs and scripts use rsync to provide hardlinked backups. See, for example, Mike Rubel's site (http://www.mikerubel.org/computers/rsync_snapshots), JW Schultz's dirvish (http://www.dirvish.org/), Ben Escoto's rdiff-backup (http://www.nongnu.org/rdiff-backup), and John Bowman's rlbackup (http://www.math.ualberta.ca/imaging/rlbackup).

Unison is a utility that can do two-way, interactive, synchronization. See http://freshmeat.net/projects/unison. An external wrapper around rsync that maintains transfer data to enable two-way synchronization is drsync; see http://freshmeat.net/projects/drsync.

BackupPC provides many additional features, such as compressed storage, hardlinking any matching files (rather than just files with the same name), and storing special files without root privileges. But these other programs provide simple, effective and fast solutions and are definitely worthy of consideration.