I just bought a Linksys NSLU2 from CompUSA for $99. It allows you to hook up 2 USB drives and share them across the network. Additionally, it will mirror the contents of one USB drive to the other on a schedule if you configure it as such.
I bought 2 Western Digital My Book drives. 2 250 gb drives for $80 each after rebate at Circuit City. These drives will spin down after 5 minutes of inactivity so you can leave them on with no ill effects. And they look shaaaaarp stacked on top of each other!
For the hackers out there, the NSLU2 runs Linux and can be modified with custom Linux installations to do a lot more. My goal is to configure it to back up to my Amazon S3 Internet storage account. More on that as I make progress.