As interesting as this discussion is, none of this particularly rings as 'cloud' to me. Although I'm well aware the definition is not set, cloud implies to me pay-as-you-go virtualization on the internet manageable via API. The whole point is scaling up and down on demand, only paying what you use, and letting someone else worry about the hardware.
In the extreme form, you build a highly-available self-healing auto-scaling infrastructure on virtual machines that could fail at any moment.
Although some will disagree with me, it's my opinion that you don't buy a cloud. Concepts like a 'private cloud' are flawed. That's just a datacenter with virtualization. You're still paying up-front. Nothing wrong with building a cloud-like setup on your own hardware, but it's a different beast. Have a look at Eucalyptus, Ubuntu Enterprise Cloud, Openstack for things like this. Great way to get familiar with the technology and to test stuff out.
David appears to want regular 'old school' virtualization on consumer hardware. Many systems provide this. For example, VMware ESXi, Microsoft Hyper-V, XenServer for the more serious testing. VMware Player, VMware Workstation, Virtualbox, Xen, KVM of you just want to test it on your desktop computer.
For example, I'm currently modeling a clustering environment by using Vagrant and Puppet to automatically build Virtualbox servers on my laptop. Good enough for figuring stuff out.
Don't go buying expensive hardware though to test cloud setups. Just set up a few actual cloud servers at Amazon EC2 or Rackspace and kill them when you're done. Should cost you just a few Euros/Dollars and you're actually testing The Real Thing.