The Multi-Display USB 2.0 to DVI/VGA or HDMI Adapter from Sabrent allows you to add up to six additional displays to your computer. It features a built-in DVI-I port and includes adapters so you can connect DVI, VGA or HDMI displays. The driver software supports Windows and Mac OS X, and the device can be used with most display types, including computer monitors, HDTVs, projectors, and even some legacy CRT displays.
The advantage of using a USB adapter over a graphics card is that there is no internal hardware to install, and the device can be used even on computers, such as all-in-ones and laptops, that don't offer expandability. Depending on your computer's display settings, the monitors can be grouped to create a single, extended desktop for increased work space, or set to mirroring so that same image can be shown on more than one display, ideal for giving PowerPoint presentations.
Note: This device will not provide the kind of 2D and 3D acceleration required for graphics-intensive processes such as many video games. It will play back video, however smooth video playback is not assured when played back across multiple monitors do to potential hardware and software limitations of the host computer.