CloneKeys works on the Mac.

Here's what I do:

3 WoWs on the Mac, broadcasted with CloneKeys

2 WoWs on a PC, broadcasted with Keyclone

Broadcasting between the machines I handle with Synergy. (Mac broadcasts to PC.)

I used to run all 5 on the Mac, but even though I was able to maintain 30fps on all windows, it got very choppy at times and my 4 CPUs were constantly pegged at near 100%.

CloneKeys (Mac only) - http://code.google.com/p/clonekeys/

Keyclone (PC only) - http://solidice.com/keyclone/index.html

And you'll need one of these, depending on which machine is your "main".
Synergy w/ Broadcasting (Mac server) - http://mendax.org/?p=577
Synergy w/ Broadcasting (PC server) - http://www.dual-boxing.com/SynergyWBroad.zip

The non-main machine(s) uses the normal synergy client, which can be found at: http://synergy2.sourceforge.net/