Amazon Simple Storage Service (Amazon S3) is the storage solution in the cloud from Amazon. Amazon S3 can be used for storing data (any amount) and the best part is that you only pay what you use (storage + traffic).

You can use S3 for backup or you can host files for your website there (like Twitter does which stores there images).

Setting up and account is easy but Amazon does not provide any tools to upload your files. So on the market there are many 3rd party clients which will help you use S3.

(soon)