Hi,
you can register for your free license key here: http://www.vmware.com/go/get-free-esxi
But be aware of the fact that the free license has feature restrictions and won't deliver you all the features you can test during the initial 60 days evaluation mode.
To learn more about the restrictions and the free Hypervisor visit this page: http://www.vmware.com/products/vsphere-hypervisor/overview.html
Regards
