I take some things for granted. I shouldn't.

x2x (which I'm glad to see is back in development!) is sort of a software KVM for X11. It allows you to map other X displays (with permissions, of course) as extensions of your existing display.

What does that mean? It means you can have two (or more) separate machines running X and mouse/KB out the (for example) left side of One into the right side of Two and interact as if it was the same machine. I've run x2x for a long time, I just forget that other people might not know about it. It's the best.

There's also XMX which allows you to run one instance of an application and display it on multiple displays which are all interactive. (Think of it as GNU Screen's multi-user sessions.)

Now there's Guievict which allows you to move a running X11 application between two displays with no forethought. It's similar to xmove but without the proxying.