Well, this may not irritate your co-workers, but this drives a guy I work with up the wall. We have 5 pc's on each of our desks, and our offices are next to each other. If I hear him leave his office, I'll sneak next door and put this on all his pc's running at once. He looses it!!!
I know you've probably seen this since it's as old as the interweb itself, but this is it...
P.S. I always leave the one on the left 'off'