You've all heard of Amazon's Simple Storage Service by now. If not then check it out and sign up for own Web Services account.
Amazon S3 gives you access to scalable storage for 15c per GB per month, with a 10c per GB data transfer cost.
Assuming you just use the service to upload data for backup purposes you are getting high performance, scalable storage for 25c per GB per month. All charged to your existing Amazon account.
There are a bunch of utilities popping up to help with the transfer of data. One such program is Jungle Disk (cost $20). Once configured, it basically sets up a new drive mapping to your Amazon S3 storage.
Simply drag and drop files onto the drive (J drive on my machine) and it automatically copies them up to Amazon.
It can be set to do backups automatically as well (eg I have set it to copy stuff up in the early morning hours).
There are numerous other online backup services available, and some work out to be cheaper if you have very large data storage requirements (eg >20GB).
However I particularly like the simplicity of the Amazon/Jungle Disk combination. In particular I like that Michele and I both have J drive's on our machines pointing to the one S3 account. I can pull down a backup of Michele's files onto my machine if required (and vice versa).
It is also easy to share files using this method (although there are cheaper methods of achieving that requirement I must admit). But paying a few bucks a month to have the security and ease of Amazon is a small price to pay.
(Jungle Disk also offer add-on services such as Web based access, you can find out more here.)