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:

https://www.altaro.com/vmware/esxi-free/