I've recently started using a second PC at work which has proven useful. In order to simplify using two PCs simultaneously, I'm using some software called RemoteD in order to share the mouse and keyboard across both machines. RemoteD appears to have been written internally at the company I work for so isn't on the web, but it's similar to Synergy in that you are able to drag the mouse off the edge of the screen on your main PC and it will appear instantly on the other screen and behave as though it were connected to that machine. Drag it back and the mouse focus is back on the main machine. The keyboard focus will also change to the PC that the mouse is active on (though you still need a keyboard plugged into the second machine in order to get through the Windows Login screen). Some software of this kind even supports copying the clipboard from one machine to another.
I'm finding this way of working is very handy to get work done on one machine whilst the other is crunching away on something that'll take a long time, but it has got me thinking that this kind of machine sharing could be done in a better way. If there was a single (virtual?) instance of the OS shared across both machines you would be able to simply drag entire applications from one machine to the other just as you can across multiple screens on one machine and it would use up CPU/memory on the machine you drag it to.
I believe this kind of thing is already somewhat possible using virtual machines for each application and using a system that allows the images to be migrated live (as used in data centre replication), but I've never heard of the system being so closely integrated with the desktop in such a way. I'm not claiming for a second this would be trivial to implement, but it would be rather cool - especially if you can add/remove machines from the array at will. For example you could bring your laptop to your desktop PC, connect them up and drag your IDE from one to the other in an instant (or however long it takes to copy the memory space of the app + related files across a wireless connection), then take the laptop off somewhere else.
In the meantime, if anyone can evangelise their favourite Synergy-type software I'm open to recommendations.