The Elastic Block Storage (EBS) facility of Amazon's EC2 world is basically a mountable, persisted partition. It was built as a direct result of the fact that when you terminate an instance on EC2, all the data that you had previously stored is permanently lost.
But EBS is more than just persisted data. It is a whole mechanism to allow for quickly taking snapshots of your data partition and then mounting that partition elsewhere. Take for example, having your database running on an EBS partition. You can snapshot that partition and mount that new clone on another machine. Quick and easy.
Amazon were quick to exploit this feature and offer publicly blessed pre-configured datasets. For example, ever fancied having the WikiPedia dataset available to your application? Or maybe you would prefer to have all the US Addresses mapping information?
At the time of writing there are 23 public available datasets for you to quickly mount onto any running EC2 instance you have spun up.
Expect this number to grow exponentially after today.
Amazon have released the ability to share your EBS snapshots to other users within the their eco-system. This means any data people are sitting on can now be quickly consumed by your application directly. This opens up a whole world of new possibilities. For example, in the forums, members of the OpenStreetMap are discussing the logistics of offering up their 60GB database to the wider community.
For more information on public EBS volumes head over to Amazon.