Xantfarm was a particular favorite of mine. Made coming into work on a Monday morning kind of fun, to see what the ants had been up to
# Xwayland :2 &
# export DISPLAY=:2
# ./xantfarmBack in the day, it wasn't uncommon that the security configuration of an X workstation was open to external windows/display. e.g. you could run a program on your system, but target your neighbor's display.
I think universities did this to make it easier to help/troubleshoot a workstation remotely, but once you figured out the commands, xroach was one of my favorites to run on another machine.