The Enomalism Amazon EC2 Migration Module supports bi-directional migration of virtual machine images to and from Amazon EC2. In other words, you can easily and transparently move running images back and forth between your local Xen-based computing environment and Amazon EC2.
Using this tool you can do all sorts of interesting things including creation of hot spares, backup snapshots, load balancing, and access to additional compute capacity to handle sudden, unanticipated bursts of traffic. The product also converts between EC2 AMI's, VMWare images, and the Enomalism Xen format using command-line tools. There's support for VMcasting, and plans are afoot to support and run Windows, Solaris, and BSD Unix in the future.
Available in standard ($295, 5 EC2 instances), Professional ($395, 15 EC2 instances), and Enterprise ($1495, unlimited EC2 instances).
Check it out!