John,

It is possible to utilize the free version of ESXi. It does come with limitations however (ie. 8 vCPUs per VM, max 2 physical sockets, can't be added to vCenter, etc). That first link will allow you to register for and obtain a license key for the free ESXi. As Raul mentioned, when you first install ESXi, it will run in Evaluation mode for 60 days. During this 60 days, ESXi will run will all the features of Enterprise Plus. Before the evaluation period ends, you would need to enter a license key to not lose functionality. If you obtained a free license, you would apply that to your host and continue running on the free feature set.

This article provides a good explanation.