I recently came across this cool chart put together by the team at VC firm Bessemer Venture Partners. They plot what they consider the top private cloud companies grouped by functional area (marketing, sales, finance, HR, etc.).
The team at BVP also has an interesting Cloud Computing Index they keep up to date which tracks over 40 publicly traded cloud companies.
Check out the chart and the index at http://www.bvp.com/cloud or, click on the image below.
BVP Cloudscape
Startup Web Guide 
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