Everyone seems to be comfortable with the idea that Amazon is the most popular cloud infrastructure provider. But I’m often asked who is second.
I’ll typically respond by mentioning Rackspace as a possible second, but will comment that there seems to be a huge gulf between Amazon and whoever number two actually is. Based on data recently published in the Jack of all Clouds “State of the Cloud” survey, Rackspace is clearly number two and the gap isn’t actually all that large.
The survey’s methodology doesn’t measure non-Web uses of IaaS, which I suspect would increase Amazon’s lead by quite a bit.

Sam Charrington on Cloud Computing