Virtual Monitor "Off Screen" to Control Remote PC I'm successfully sharing my Ubuntu Desktop with my Mac Book, using VNC.

My Mac Book Pro is on the right, my external monitor is in the center, and I have an Ubuntu Laptop on the left as in this image:



While using the Mac, I'd like to be able to slide my mouse to the left of my centre external monitor, so I can think of the Ubuntu system as an additional monitor, as it were. 

I have been able to do this from within Ubuntu in the past, by setting up a virtual monitor using "x2x -south -to :1"

I'm curious as to whether this functionality is possible using my Mac.

UPDATE:

I've found the following which are close, but "no-cigar"
https://superuser.com/questions/276766/create-virtual-external-display-for-vnc-control

I've found the following software that might work, though not free:
http://symless.com/synergy/
Synergy is still open source (therefore free), though they don't really go to great lengths to make that known.

https://github.com/symless/synergy

Synergy works pretty well from what I remember.