But due to session cookies, it's easier to download via browser, then scp or sftp the ISO to your server.
To download the of VMware vSphere Hypervisor (ESXi) for free (free hypervisor) or for a trial, follow these steps. There are two main sources depending on what you need.
VMware Customer Connect - ESXi Free
When you reach the download page, you will typically see several file types:
This is the proper, legal, free edition for a single server with up to 8 vCPUs per VM. You get unlimited VMs.