There are many ways for a networked Arduino or compatible board to communicate with the outside world, and one of the most common methods is with HTML thanks to the simple code on offer. However for a deeper integration into what is being coined the "Internet of Things" the choice of a standard data protocol would be preferable. One example of this is MQTT - "Message Queue Telemetry Transport", and a tutorial on how to use this with an Arduino has been presented by Chris Larson.
By following Chris' guide you can connect an Ethernet or GSM-enabled Arduino to use an online MQTT client by the name of 2lemetry and get connected for data transfer and so on. Apart from the guide, all the libraries and example sketches are provided so you can get started - so visit Chris' website for the tutorial. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
When putting together your next Internet-enabled Arduino project - save time, space and money with the Freetronics EtherTen. Apart from being fully Arduino Uno-compatible, it has onboard Ethernet, microSD socket, full USB interface (so you don't need a costly FTDI cable just to upload a sketch!) and supports optional Power-over-Ethernet.