While searching for better software for assisting Multi-Monitor setups without packing a dozen video cards into one PC I came across MaxiVista. Although similar its not free like Synergy. But for thirty bucks it looks like you get a lot fore features. They tout it is designed to use any laptop as a monitor, but desktops should work similar. Too bad there is a 4 PC limit.
MaxiVista turns any spare Laptop or Tablet PC into a second monitor for your primary PC. Program windows can be extended across both PC’s screens, while the programs are still all controlled from the primary PC. Connect up to four PCs via Ethernet, wireless LAN, FireWire, or USB and enjoy a giant desktop real estate of up to 7,680 x 1,600 pixels. The extra desktop space helps to avoid switching back and forth between overlapping windows. Additionally, MaxiVista allows to control up to four PCs with a single mouse and keyboard. Just drag the mouse cursor to the PC you wish to control. The clipboard of all computers will be synchronized as well. MaxiVista can also mirror the display contents of your primary PC to any other PC via a network connection without signal loss. MaxiVista is based only on software. No need to buy graphics cards, monitors or keyboard mouse switches.