I simply rsync my entire server (over ssh) to a large disk on another box. Super easy to set up and the backed up data doesn't need any special software to read/restore, beyond standard 'nix tools.
A proper backup involves copies that cannot be modified after they are made (i.e. a snapshot after the backup has run if using disk, or for free on tape).
As it was ambiguous, I figured you must have been talking about the more likely scenario whereby files become corrupt on the live server, unknown to the either users or admins. Over time, these corrupt files then make their way through all generations of backup until no good copies remain.
There's nothing special about an rsync/ssh solution that precludes the backup server from creating a read-only copy of each and any backup.
I solve this by supplementing the daily networked backups with weekly manual backups to an external hard drive that remains disconnected at all other times.