Hovering a mouse in the corner of your screen can be used to launch an application. To do it, get xautolock.

sudo apt-get install xautolock

And place a command like this in one of your autostart files:

xautolock -locker "screenruler" -corners 0+00 -cornerdelay 1 &

This will launch screenruler after 1 second of mouse hovering in the upper right corner.