Amazon Simple DB

Correction: In a comment, Justin Crites points out, correctly, that Simple DB does not offer 'relational' database functionality, in the sense that it is not an RDBMS. While this is true, I think Simple DB clearly offers functionality which many people get, perhaps inappropriately, from a relational database system.

Amazon continue to expand their excellent infrastructure web services. Simple DB - relational databases in the cloud. I'm definitely going to have a look at this - clustering/replicating databases for resilience and performance was extraordinarily difficult when I was developing and supporting enterprise systems a couple of years ago. In fact, more often than not, we'd cluster and load-balance all the layers in the enterprise stack except for the database, and then just rely on a singe very expensive box running Oracle Enterprise 9i or whatever the equivalent is nowadays.

Like the other Amazon infrastructure services ( S3, EC2 you pay for what you use. Linear costs. Simple. Brilliant.