It is allowed to re-sell Piwik and provide it for free, or for a fee. In fact, we maintain a list of Official Hosting Piwik partners which we recommended. If you are providing a service reusing Piwik, you must also reuse the word Piwik with care. Please see the Piwik Trademark page for more details about how to reuse the name Piwik in your modified version of Piwik. In particular, it is not allowed to use Piwik in the URL domain name. We would love to know if you create something interesting using Piwik: drop us a message.