We have many choices for backups on Linux and *BSD systems: Bacula, Amanda, Arkeia, and more. My pick is Rsnapshot, which is based on the excellent Rsync file mirroring utility.
Rsnapshot uses Unix hard links to effectively keep a full backup for each snapshot while consuming only the network bandwidth and disk space of a differential backup. The backups are saved to regular file systems on the backup server's hard drives, not tapes, so there's no media to change out. Because Rsnapshot uses common Unix tools, such as Perl, Rsync, and hard links, it runs on most any Unix-ish operating system -- even Mac OS X, Solaris, and Irix.