FreakZ is an open source Zigbee stack licensed under the modified BSD. It uses intellectual property in the form of the Zigbee 2007 specification which is available from the Zigbee Alliance and can be downloaded from their webpage. There is an additional restriction imposed on the software in which it is subject to the Terms of Use of the Zigbee Specification. The TOU can be found at the front of the Zigbee spec.