In my old investment advisory firm, we had to have a backup plan that passed SEC scrutiny. We had a server with RAID5 which just meant that we had 5 mirrored hard drives. We actually had the main drive fail twice in 14 years and it was easy to remove the bad drive and hook up one of the mirrored drives. Over the years, the cost of this setup has continued to decline.

Then we rented space on a server at a data center (similar to what Mozy and others use) and backed up to that server every night. These backups were incremental so that if a file was deleted or overwritten, we could retrieve the file from a previous day.

So effectively we had an on-site backup in case of hardware failure and a dedicated off-site backup to restore deleted or corrupted files or in the case of theft or fire in the building (or even a hurricane).