Although there are a plethora of Arduino-web control services, sometimes the simplest options are often the best. With this in mind, if you have a static IP address or are within an Intranet the following simple project by Instructables user 'jamesrww' will be of interest. He has described quite plainly how to control an LED (and therefore a digital I/O pin) from a simple piece of HTML - and also to read the pin status. This would make a simple remote control for intra-office tomfoolery or more serious uses such as door lock control.
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.