ESXi registration is free, although the steps to getting a license key are not necessarily straight-forward. You will need to register a free account with VMware in order to get a license key.

Get a license key

1. Log on to VMware.com and register and account. The current link is:
2. Log in using your account
3. Go to vSphere download page. The current link for version 5.1 is:
4. Scroll down to “License Information” section

Now that you have your key, you will need to assign it to your ESXi host

Assign the key to the host

1. Log on to host using vSphere
2. Go to “Inventory”
3. Highlight the host’s IP address
4. Go to “Configuration” tab
5. Click on “Licensed Features” under “Software” section
6. Click “Edit…” on the right hand side

7. Select “Assign a new license key to this host”
8. Click “Enter Key…”
9. Copy/paste your key here
10. Click “OK”
11. You should now see the word “Never” in the “Expires” section