I use Synergy too - and the multiple inputs of my monitors to allow reconfiguring of which PC has two screens.
eg. Linux 1 running linux on the left, workstation in the middle with two monitors connected with DVI cables, laptop on the right with Windows.
Each screen has an HDMI, DVI and VGA port - switchable by pressing the input button on the screen.
Each laptop is also connected to the closest monitor, using HDMI cables.
Synergy server runs on the workstation, with the mouse and keyboard connected to it.
Each laptop is a synergy client.
so all together 3 pcs, 4 screens, 2 different operating systems.
I can flip between using a single screen or dual screens with the laptops, (which of course means using only a single screen or no screens for the workstation)
in practice, I don't usually need to bother dual-screening with the linux laptop, as I tend to use it's screen to edit files on the workstation anyway, or just use it to browse documentation on the web - but I do find it useful to sometimes dual screen the windows laptop at the expense of one of the workstation screens, such as when I am doing a lot of windows development and debugging. Of course if you have 3 or 4 monitors it's easy to just insert that in the mix and not have to flip screens at all - and if you have a few more old crusty laptops or a mac or something they can very effectively become part of this setup by just adding them as additional Synergy clients.
The only downside is that if you have the laptops set up in multi monitor mode when the workstation has use of both screens, you have to move the cursor across the 'hidden' screen - but of course you can just switch back to single display mode for the laptops if you aren't going to be switching displays back and forth much, and you get used to the 'blank' spot (in the mouse's travel) pretty quick .
Of course you can also copy and paste text back and forth between all three machines, which I don't believe you could do with a traditional KVM setup.
On Linux, as soon as my laptop wakes (from sleep or hibernate) , it happily reconnects to this config, and works without having to reset anything.
Unfortunately with Windows, sometimes when it comes out of sleep, you have to stop and restart the synergy client for it to reconnect, though it seems to have fewer issues with hibernate.
You can also layer in something like VNC or remote desktop into this mix if you need to connect to additional desktops too, of course - but the main thing is you have a whole lot of screens, a whole lot of PCs and only one keyboard and mouse to rule them all.
Total cost: $0