Using an illuminated cube received at a job fair, Jamie Matthews used an Arduino to control the cube for a useful purpose, as an automated GMail notifier. The Arduino is connected to a PC via the usual USB cable, and a Python script running on the PC that checks for new mail and sends characters to the Arduino that are interpreted as "new mail", etc.
Although this may seem like a simple project, it is an excellent and inexpensive demonstration of how to a PC can control devices via an Arduino and web-based information. For example, instead of lighting a cube, a servo could wave a flag up when mail arrives. You're only limited by your imagination. So to get started, review the details and Python code here. And we're on twitter, so follow us for news and product updates as well.
So if this sort of project intrigues you and you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: