VMware vSphere Hypervisor is based on VMware ESXi = Free
"There are two vStart setups start, which are configurations designed to support 100 or 200 guest slices running atop VMware's ESXi 4.1 hypervisor. This is the freebie version of the hypervisor from VMware; "
Facts: VMware vSphere Hypervisor is based on VMware ESXi and is Free.
ESXi is not Free.
"the machines are also configured with a trial version of the vSphere Enterprise stack, which includes VMotion virtual machine live migration, high availability failover, load balancing, server power management, and other key features that make server virtualization useful."
Standard installation of vCenter enables those features for 60 days.