1) Amazon AWS
All four of these have all the basic building blocks to develop, host, and scale web applications. Amazon AWS is a bit differnet as it exposes virtial machines (EC2) instances where as the other are more abstract and expose application instances. Therefore Amazon AWS can be used to build a complete VDC (Virtual Datacentre) to host applications as well back office services such as email, active directory etc.
Services such as GoGrid ultimately still bring physical servers into the picture so they have been discounted.
Are there any others worth talking about?