Thanks at least in part to the name-brand recognition of parent company Amazon.com, Amazon Web Services is one of the most popular (or at least one of the most well-known) cloud-computing systems out there. Small wonder AWS sports a bevy of third-party tools to aid the use of its services. Sometimes this comes in the form of another service -- a kind of mediator between you and AWS.
Elastra's Cloud Server is one example of such a system. An application that uses MySQL, Ingres, or EnterpriseDB can be deployed into Amazon EC2, with its data stored in Amazon S3. Failover, replication, and elastic clustering (the ability to add or delete nodes based on demand) are all handled automatically by Elastra. The application side of things is more your responsibility, but you're essentially given a Linux 2.6.16 platform in one of three basic instance configurations that you can write apps in.
Another mediator is RightScale, which also offers a tiered approach to elasticity. A free developer edition lets you get your feet wet right away with 10 free hours of EC2 usage. The other tiers (starting at $500 a month) allow you access to expanded functionality -- scalable batch processing, scripting, multiple server deployment, and for-pay support and consulting.