We use Win2VNC here, or at least used to. Our old security DVR software was a pig, so our guards had two high power workstations. To make it simpler for one person to operate, and minimize desk clutter, we used Win2VNC to bring the controls of PC #2 over to PC #1. The clients just have to have a good old VNC client installed. I knew of Synergy before Win2VNC. I'm not sure why I opted for Win2VNC besides the familiarity with, and simplicity of, VNC.
There's also X2VNC for the linux folks; have not tried that myself though.