Since I converted to the Mac two months ago, I've been playing Russian Roulette. I haven't had a good system for doing backups. But that changed Sunday when I discovered the Amazon S3 online storage service which, when used with the free software Jungle Disk, provides a cheap, easy way to back up your Windows, Mac, or Linux computer online. Storage prices for Amazon S3 are staggeringly cheap.

We've written about S3 before; it's primarily designed as a storage service for people developing online applications, but there's no reason you can't use it for personal storage and backup.

Amazon charges a shockingly inexpensive $0.15 cents per month per gigabyte of storage, and $0.20 per gigabyte transferred. I have 30+ GB of data that needs backing up on my Mac, that means I'll be able to back it up for roughly 11 bucks the first month, and probably about six or seven dollars for each subsequent month.

I'm not a software developer, and so Jungle Disk is essential to allow me to use S3; it's a desktop interface for S3 with versions available for the Mac, Windows, and Linux.

Jungle Disk doesn't just manage backup, it also allows you to address S3 as if it were just another local disk drive. So you can use S3 as online storage for files you share between multiple PCs.

It's going to take days to transfer my 30+ GB of data to S3, but that's OK. It's not like I actually have to pay attention to the process and do anything to move it along. I just set up Jungle Disk and forget it. It runs automatically, at regular intervals that can be set by the user: Weekly, daily, every six hours, every five minutes, etc. You can also set it to run manually.

S3 isn't the only online backup option available; Serdar Yegulalp reviews five of them.

Based on personal experience, the review, and a little research of my own, I think there are only three online backup services worth using: S3, Mozy, and Carbonite.

Mozy and Carbonite are both Windows-only, and that means that, for Mac users like, S3 is the winner by Hobson's choice.

Why do I like those three backup services? Because they offer unlimited storage at a flat rate or -- in the case of S3 -- a per-gigabyte rate that's so cheap it might as well be unlimited.

Many other online backup services set limits on how much you can back up. I just don't have enough time in my life to figure out which of my files should be backed up and which are expendable. I want to just back up all my data files, and then move on.

Other online backup services are unacceptable because they charge exorbitant data fees. For example, AT&T Online Vault wants $5.95 per month for the first 2 GB, and then $2 per month per GB after that. That's more than 13 times as much as Amazon charges!

Update, 7:40 PM EDT: In the comments below, "jrk" points out that Mozy for the Mac is in public beta. Glad to hear it! And "roger" says XDrive has "5 gb storage free and 50 gb for 9.95 month." XDrive is a great service; I use it myself for sharing files between computers, and passing along documents too big to send as e-mail attachments. But that 50 GB quota means that I will, one day, have to start thinking about which files I want to back up and which are expendable. And I don't have time for that, life is too short.

I've experienced about one hard disk crash every decade. That's not frequent, but it is potentially catastrophic. If you're not backed up, your data, including important work files and financial records, is gone forever.

Local backups -- a USB or Firewire drive sitting on your desktop -- are useful, but they're not enough. If there's a fire or some other disaster in your home or office, your local backups might well be destroyed along with your PC. Until recently, home and small business users filled the need for remote backup by cutting disks and storing office backups at home, and home backups at the office. Or they gave the backups to a friend to hold. I know one guy who rents a safe-deposit box for his backups. That's got a certain coolness factor, but online backup is easier.