First sign up for a new account here
https://customerconnect.vmware.com/account-registration
Then go to this link depending on what version you want to register for
Version 7:
https://my.vmware.com/en/web/vmware/evalcenter?p=free-esxi7
Version 6:
https://my.vmware.com/en/web/vmware/evalcenter?p=free-esxi6
Version 5:
https://my.vmware.com/en/web/vmware/evalcenter?p=free-esxi5
References:
