The esxi (standalone server) is free, but it must be licensed. If not, it installs as 60 days trial, which expires after 2 month.
How-to receive and apply VMware Free License to VMware ESXi 6.0?
1. Download the VMware Hypervisor from this page (you’ll need to create an account if you don’t have one – it’s free). For esxi 5 use this page.