Amazon S3 Storage Service provides a really great deal on massive online storage.
Amazon have spent a lot of money designing a massive online data storage center with the principles of speed, reliability, cost effectiveness, simplicity and scalability. The storage center is based on a lot of cheap components, but data is kept safe with replication.
You only pay for storage and bandwith that you use. There are no setup costs and no minimum fee.
Monthly prices at time of writing were:
$0.15 per Gig of storage used.
$0.20 per Gig of data transferred.
Amazon S3 is aimed at developers and accessed via a simple REST/SOAP API.
This kind of storage would work really well for sites with variable storage or bandwith. It is worth comparing the price to something like a dedicated server if you are looking for a online data storage.