Cloud Computing: What does it take to setup a Cloud

These are a few open source cloud servers:

• http://www.eucalyptus.cloud
• http://www.enomaly.com/
• http://sourceforge.net/projects/eyeos/ (semi-"cloud")

You should check out Eucalyptus. The goal of Eucalyptus is to allow you to setup your own private cloud, similar to Amazon's EC2.