Depends on how much data you're talking about. I use RAID5 arrays for my data, but even RAID is really considered a proper "backup" solution, just a redundancy in case of not quite catastrophic failure. For really critical files -> digital photos, home movies, finacial records, etc, I do periodic dumps to DVD just as a sanity check. I have had a RAID5 array go down completely so I like that extra sense of security.
Other options are tape backup, periodic disk mirroring, etc.
