Just ran across this yesterday, been goofing around with it.
http://www.maxivista.com/.
It has a client and server app which sets up a virtual display over a network that acts as a secondary display on the primary machine. It's very handy if like me you have a laptop setup on your desk as you can turn it into a second display (like adding another monitor) without any additional hardware. It's shareware with a 14 day or 50 startup trial period with a purchase price of 30 to 70 dollars depending on what you want for extended functionality. The only real drawback I could maybe see is in network traffic, but even when I tried it on a 10 Mbps connection it worked fine.