1. A cloud computing system, the system comprising: a computing device configured to run virtual machine instances, the computing device including a hypervisor program for managing the virtual machine instances; a customer virtual machine instance run by the hypervisor program on the computing device; and a grid virtual machine instance run by the hypervisor program on the computing device, wherein the grid virtual machine instance is configured to run only when a resource of the computing device is not being utilized by the customer virtual machine instance.