It is no longer available.
Once the evaluation period expires, the VMs will remain in their current state, but you will not be able to do administration tasks, like e.g. powering on VMs.
André
@jasperli Just to second @scott28tt's suggestion, if this is for non production use, then the VMUG Advantage Program is the way to go. You can get 1 year subscription for $210 which will give you access to a whole bunch of software not just ESXi on 365 day licences. You also get a slight discount if you buy 3 years.
If it is for Production use, then there are 4 options available, the cheapest of which is $800 per year for 1 server, see below