As you may have seen or heard on reddit or elsewhere, ESXi free is no longer available. So what does that mean?
First of all, if you are already running ESXi free version, don't worry. The license you have for that is perpetual, meaning, that you can use it for as long as you want as long as you don't lose your license key as there probably wont be any means to recover it once lost. So better back it up now or even print it out.
If you need another key or don't have one yet, you're a bit out of luck, as unfortunately you will not be able to acquire a new key now. You can still use ESXi in evaluation mode for up to 60 days or use any other license you might have.
So what are your options now? Let's divide these by use case:
Lab use
If you have used vSphere ESXi free in a lab environment to test new things and hone your skills then you have several options. Firstly, what did you use your lab for in the first place? If your answer is to try out VMware products, maybe even learn something to use at work or to learn for an upcoming certification, then you absolutely want to continue using VMware products there.
Now in theory you could just run it in evaluation mode and re-install it every 60 days once the evaluation runs out (you can even do so automatically) but realistically no one has time for that and it is very tedious.
You can also buy a license but that's also not very realistic as a full-fledged license costs a lot for lab use and even the cheapest "Essentials Plus Kit" license it expensive and doesn't offer the features you probably want to use in the lab.
So in my opinion the best option is to use VMUG Advantage. VMUG is the VMware User group, a global organization for and by VMware users that aims on bringing knowledge to you by organizing events, sessions and trainings. A standard membership is free, but for only 210$ per year (as of now) you get the Advantage membership with some benefits:
Not only do you get discounts for trainings and certifications, you also get a 365 days evaluation license of most VMware products, which include vSphere Enterprise Plus, vCenter, NSX, Horizon and many more. Just keep in mind, these licenses are ONLY for lab and testing environments and can not be used in anything that's hosting productive workloads.
Now if you don't use your lab for anything VMware related you can look at other free hypervisors out there, such as Proxmox or XCP-NG. This will obviously require you to migrate all your workloads over, so if you like your current solution, in my opinion VMUG Advantage is worth it for that.
Production workload
If you have used your free ESXi hypervisor for anything production related you unfortunately can not use the above solutions, other than switching to another hypervisor. If you don't want to switch you will need to buy ESXi from now on. If you are using ESXi Free as your only Hypervisor and don't have any other VMware products currently, it might not be worth it buying it. Although, with the cheapest option the "Essentials Plus Kit" you get 96 cores for up to 3 servers and access to vCenter server, that is probably more than you need.
So realistically the only option would be to switch to something else, or if you use other VMware products already and have valid licensing in place see if you can somehow accommodate the workloads on that machine somewhere else.