Lnbackup is a hardlink backup system for hard drives. Lnbackup operates in a way similar to the '--link-dest' switch in rsync. It creates incremental backups using hardlinks so that each backup seems like a full backup. Additionaly it can make (using hardlinks) bootable mirror from the latest backup. Obviously the target filesystem of lnbackup needs to support hardlinks. It's run on ~200 servers for several years and it is considered stable. Read the man page for more information.