Amazon just announced a new file storage system for EC2 instances called Elastic File System (EFS). This can be shared across multiple EC2 instances in your VPC. It is a NFSv4 file system. It auto-scales up/down based on use. Therefore it's pay-as-go storage. So instead of pre-allocating a drive based on your expected usage, you can just let EFS scale as you need it.

As of now it's in Preview mode and available only in the US West (Oregon) Region.