I used the link that IBM_india has provided to download the ESXi 6. I think a.p. is correct, that the ESXi component included in the vSphere trial is identical to the free Hypervisor. Actually I had used my Hypervisor license key to activate the ESXi 5.5 from vSphere trial about one year ago. I totally forgot this before IBM_india posted the link.
The link actually points to a newer build version of ESXi, which is a good thing. The only concern is that I have to start a trial, which may trigger some sale contact that I don't really need.
Anyway, I sent an email to the VMWare website contact to let them know there is an issue related to the free Hypervisor downloads. When I checked again today, the page has changed, though still not working. I think they are working on it.
Thank you to everyone.