can't believe nobody mentioned redundant RAID drives?
Not a backup, granted, but a HD WILL die at some point - why risk putting your backup abilities to the test and all the pain, headaches and downtime until you can restore from backup?
Yes backup is essential, and offsite backup at that (not onto a second HD on the server...what if a tornado hit the DC?), but RAID1 is at the least the minimum in combo with daily offsite backups...
If anyone is interested in $1/GB remote backups to a tier 1 DC, hit me up on icq 283914791