There is a key broadcasting version of Synergy linked in Ellay's thread here:
http://www.dual-boxing.com/forums2/viewtopic.php?t=15

Ellay says:
I personally use this application and love its functionality. Allows you to seamlessly transition from 1 PC to another simply by moving your mouse to that screen or through the use of hotkeys. Once you've moved over to another PC your keyboard and mouse have complete functionality.

Each PC does require it's own monitor, it just enables you to control all with 1 keyboard and mouse. Setup time does take a bit, but is well worth the time spent.

Stealthy Says:
I started using this becuase multibox was very unstable on my machine and haven't looked back. Mousing between the 5 screens is a breeze, and it makes the collection quests a lot less tedious. Key broadcasting can be toggled on and off with any key you set it up as (I use the ` key).
Just remember when setting up your monitor configuration, you must not only specify that 'monitor 1' is to the left of 'monitor 2', but also that 'monitor 2' its to the right of 'monitor 1'. Otherwise your mouse will get stuck in one screen when you move it over.

Cheers,
Stealthy