A couple of times in the past I've thought about using other computers/tablets as additional displays for my main computer.

This time I actually found a way to do it.

xrandr can be used to create a new monitor without the need for an actual monitor, I used the VIRTUAL1 output and was pleased to see a VIRTUAL2 pop into existence when I did. Then x11vnc with an appropriate -clip can make a VNC server for that bit of the virtual desktop.

I tried both a spare laptop and my tablet as extra displays and it seems to work well with either.

The tablet as extra screen will be quite nice when I'm travelling.