Being able to work on a double screen is particularly handy, however this usually means that you're restricted to working on a desktop computer rather than a laptop dual screen.
The DuoScreen from Nomadz describes itself as "Your laptop's missing half," since it allows you to securely connect a second screen right onto your laptop computer. This might seem rather cumbersome, but the system is designed so that the attachment is harnessed to the underside of the laptop, so that it can be easily flipped out or folded under as needed. Plus, in order to set it all up, all you need is a USB connection. The second screen on the laptop is also self-supported, which means that it can be suspended in mid-air without the need to rest directly on a surface below.