Tobii peripheral adds an eye-tracking interface to any Windows 8 computer through a USB port, but the company's only offering 5,000 units before the end of the year.The accessory attaches to the base of your laptop screen or monitor, and tracks the movement of your eyes.Whatever you may feel about Windows 8, it's sparked a number of interesting hybrid designs. Now you can count the Tobii Rex, an eye-controlled interface for Windows 8, as another innovation that works with Microsoft's latest operating system. First seen at last year's CES, the Rex is an eye-tracking peripheral that works with Tobii's proprietary Gaze interface to navigate around