Free alternatives to vSphere for ESXi (if one exists)
You can manage your ESXi VMs and hosts indefinitely with the "free" license of vSphere ESXi using the included vSphere client.
What you're not getting, for free, is the "vCenter Server" product, which allows centralized management of multiple ESX and ESXi instances from a single console.
Insofar as "practicing for certs" (assuming you're looking for a VMware certification), you're going to want to use the trial edition of the full-blown ESX product anyway, because you're going to want to practice using some of the more advanced featuers.