How are you handling the actual provisioning of the agent boxes/VMs? Are you using a specific provider/cloud, or multi-provider? Are there any providers/clouds you've found are better/worse for your use cases?

Asking because I am working on an open/standard protocol/layer for provisioning cloud resources across different providers. One of the ideas is to provide a marketplace of providers/resources via one unified/standard API, which I would allow automatic selection and provisioning of VMs (and other resources) based on price/value/feature/reputation requirements/priorities.