ESXi is a Server Hypervisor, doesn't have a gui to use as VirtualBox for example.
It depends on where you want to use it.
If you plan to use it on your notebook or desktop, consider use VMware Server.
Otherwise, ESXi is free but you have 60 days of trial of all enterprise feauture like the licensed version.
If you not plan to use enterprise feauture, you should make the registration on vmware site, to gain a free perpetual license for all the basic feauture.
Bye
Riccardo