With an Arduino and some inexpensive sensors you can experiment with and possibly recreate all manner of complex devices. One example of this is to build a simple "radar" style device, and this has been demosntrated by Ujash Patel with great success. By using an inexpensive ultrasonic distance sensor mounted on a servo, it can be rotated over the range of the servo and a distance taken for each degree.
This data can then be sent to a PC to be mapped graphically on the screen, and Ujash has used Processing to do this quite well - a screen that wouldn't look out of place in a movie or elderly submarine can be created to display the data from the sensor, for example:
This could be a lot of fun, and an interesting framework for adding "vision" to an autonomous robot. For complete details on this example, visit Ujash's Instructable page. And for more, we're on facebook, Google+, and twitter - so follow us for news and product updates as well.