You can use it as a live online network disk (it looks like a hard drive to Windows/Mac) or for backup.
The Jungledisk program is only $20 and Amazon charges very little for the online storage. I use several gigs of storage and pay about $2/month.
If you are really cheap, just set up a Gmail account and email files to yourself. Gmail is up 7 gigs of free storage.
Me, I'd use rsync to an external USB/eSATA/NAS. 1TB of storage is relatively cheap these days. Having it in your possession rather than paying some on-line service (where you really don't know what's going on behind the scenes - regardless of privacy policies) more than justifies the price of the drive.