New providers are springing up frequently, which makes evaluating them all that much harder. Some are traditional hosting providers, other offer more virtualization expertise and some have built their own management tools around their services. All this activity is a good thing, because chances are you will need more than one cloud provider by the time you get serious about this technology: some are better at hosting servers, other at providing more networking or applications infrastructure, and others at provisioning new servers.
Let's start with the traditional vendors: Microsoft is trying to capture some of this market and has a great starting place for more information, click here, including a list of hosting providers that support Hyper-V. And, Intel has its Hybrid Cloud offering. Check it out here. Intel's program is in limited beta and looking to sign up VARs who are interested in providing managed servers and want to include a variety of options, including firewall, VoIP PBX, virtual storage, and management tools. Google has its Apps and App Engine, along with a series of Web services. Finally, Amazon's Web Services have been around for many years and includes CloudWatch to manage hybrid clouds.