We just added a new feature called Devices. This allows you to register your home-brew devices and receive a globally unique MAC address. Some devices do not come with a MAC address so often the MAC address used is the one that the example source code is using. If you end up putting two Ethernet or WiFi devices on the same local network with the same MAC address, odd situations will occur and will not be obvious to troubleshoot. The Devices App from ThingSpeak can generate a random MAC address or generate a globally unique address from our address pool. We have a limit to the number of global addresses we can generate, so feel free to jump in and grab one for your Thing.
In the future, we will add more features to the Devices application. If there is interest, we are thinking of setting up a dynamic DNS service. This is so you can use a consistent URL to access your device from the Internet.