I've been using Amazon's S3 storage, it's cheaper than building your own server and maintaining it, and it's also more reliable than an external hard-drive.
http://aws.amazon.com/s3/
I'm not a huge tech-savvy person, so S3 was an easy decision for me. If you have your own domain you can also hook-up your S3 service and use it that way, as well. It's kind of like webhosting, but this service is a bit different, same concept but you don't get screwed over ($$$) by hosting companies.
I've been using it to backup data, costs me like $3/month, which is pretty cheap considering.
Here's a good blog about it, with a pretty good cost/benefit analysis.