What is ESX Server?
in application packaging
ESXI is a hypervisor (an OS that supports running VMs). Up to a certain point it is free (# of sockets and amount of RAM), but if you start using it for production you'll likely need enterprise licensing. We have a test box here that's a 2nd Gen i7 with 16GB of RAM that contains a virtual lab with 8 VMs in it. It's our test environment for software deployments and configuration changes.