After looking for various methods of interfacing an Arduino and connected hardware with a web interface over the Internet, Michele Borioli decided on using the MQTT lightweight messaging protocol for "small sensors and various devices".
After some research Michele explains the basics behind the protocol, finds us a message broker and explains the hardware required from the Arduino side of things. They also include links to the required MQTT Arduino library and ends with an example sketch of a web-controlled digital output.
If you're experimenting with Arduino and MQTT as mentioned above, save time, space and money by using 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.