I've spent an hour looking at the licensing info on the VMWare site, and also contacted a VMWare support rep who simply directed me back to the parts of the site that I'd already seen (Licensing ESXi 5.x and vCenter Server 5.x (2014295) | VMware KB), so...
I want to run a standalone ESXi 5.5 server (free version), and manage it with the free vSphere client. It's basically a testbed that I can use to explore and learn about the platform. I've installed ESXi 5.5 (having finally found a way of downloading it, which was far from straightforward!). When I launch it I get a message saying that it's a 60-day evaluation, and that I will need to enter a licence key if I want to use it after 60 days. My question is: how and where can I obtain the licence key necessary to continue running the free version of ESXi 5.5 and vSphere after 60 days?