Couple of months back, I downloaded the demo version of Maxivista and I loved it for many reasons. It includes a virtual graphics adapter which fools your primary PC into thinking that there is an additional monitor display installed. Windows enables its multi monitor capabilities and extend the desktop to an additional screen. The content of this extra screen is transmitted to your secondary PC via a standard network connection and displayed by the MaxiVista Viewer program.
Personally, I have been using it to pull out my mail client on a rarely-used-computer so that I can see mails coming in separately and effectively. Though I have not really tested with heavy resource intensive applications, I like their Low CPU utilization with the program. It claims that, "intelligently analyzes the secondary display content and only transfers changing regions between single frames. If the viewer screen content is static, the system load is down to zero! In normal use the CPU load rarely exceed 5%."
Another interesting thing was that if you already have a dual Monitor setup, it adds a third display to gain maximum viewing space. MaxiVista allows you to stretch application across both screens as it would be one single big display. You can stretch application across both screens as it would be one single big display. $49.95 for a single licence isn't that bad for such a cool application. They recently introduced a $44.95 for two licence scheme, check out more of their pricing. Or just download a demo and try it out yourself before you buy.