The Buffalo LinkStation mentioned by drew can, like the Linksys nslu2, be set up to automatically back up its main disk onto the secondary disk every night. (Might save you some time drew.)
The Buffalo received better reviews than the Linksys in terms of throughput.
Neither of these systems is as good as a RAID, but for a home user on a budget, they provide a big increase in security.
Mac users have another option: simply attach two external drives of the same size (firewire is the recommended interface as it is higher performance than USB in real throughput) and tell OSX to configure them as a RAID 1. Presto, you have redundant storage.
Want easy off-premises backup in case of fire or disaster? Buy a third disk. Fail the RAID by removing one disk (which you will now store off-premises), plug in the third disk in place of the one you removed to take off-prmemises, tell the OS to rebuild the RAID, and you are back in busniess. Rotate the on-line and the off-premises disks to ensure your off-prem disaster backup is up to date.
Finally, if you want the data on the Mac RAID to be available to others, you can share it over your local network and/or you can enable the Mac as an FTP server (it's all built into OSX) to allow remote access from the internet. You'll need to do the same trick Michael mentions in the addendum to his article if your ISP dynamically assigns you an IP address.
BTW, the cheapest way to get an external hard disk is to by a bare ATA disk drive (don't waste your money on SATA as neither USB2 nor firewire run that fast) and pop it into an external USB/firewire enclosure. I noticed Macally enclosures for about $US 50 and the price of disks just keeps coming down.