Another great technote article over at Citrix support website. Customers have been asking me this question from time to time.
So, since many of these customers probably follow this website, here is a quick how to renew the free xenserver license.

To renew a Free XenServer license, follow these steps.

1. Open XenCenter.

2. If you have XenServer licenses expiring within 30 days, the License Manager automatically open. If it does not, open it from the XenCenter menu by selecting Tools > License Manager.

3. XenServer hosts requiring renewal licenses have a check box preceding the host name, and indicate the number of days remaining until license expiration.

4. Press Select All to update all hosts.

5. Press Activate to launch the activation process.

6. Enter the requested information, and press Request Activation. The license file is emailed, within 10 minutes, to the email address you supply.

7. Upon receipt of the license file email, apply the license to each host by opening the XenCenter menu Server > Install License Key.