Pull a 4chan and do a raid. Find out his address and order a bunch of stuff for him, like $200 worth of pizza or chinese food.
Or arrive at work before he does with baby powder and a paper bag. Fill the paper with the baby powder, slip it partway under his door and then jump on the bag so the baby powder flies out everywhere in his office.
Ooorrrr piss on a plate and freeze it so that you have a thin disk of piss. Take that disk to work in a cooler or something and then before your boss gets in slide it underneath his door. It will melt and he will have a rather nasty surprise when he walks in.
Kill him or her with kindness - I've found that that totally throws most people way off and they end up shooting themselves in the foot trying to figure out the 'angle'. Just go to a nice quiet place afterwards and scream - sometimes it is easier to do than others! I've found that revenge usually only backfires on me so I gave it up quite young.
LOL. What a fun idea for a thread! We have some very evil and very creative hubbers here. I'm sure you'll get many good ideas (which could then be turned into a hub or hub series).
My suggestion is going to be typical MM psychobabble. LOL.
Kill him with Kindness Bend over backwards to be nice and helpful. Pretend like the dispute isn't even happening. It will unnerve him and diffuse the tension. You will look like a star to everyone in the office -- likely including your boss's boss.... It's like taking the high road to a whole new level.
Have one of your female friends with a sexy voice call the morning after he comes home late from work and leave a message saying that she had a great time with him last night and that they should do it again (Of course, she has to call from a private land line so that it is not traceable.). Hopefully his wife will hear it and confront him when he comes home.
Screw up the computer system or sign his mother up to an adult dating site, make her out to be some kind of deviant... send multiple magazine subscriptions to his house via the internet. Remember, revenge is a dish best served cold; so you will have too take time to acquire the proper information to do you dastardly deeds.
I have a complete and utter lack of creativity. The only thing I can think of is beat him over the head with a baseball bat or cast-iron skillet (but you'll probably be arrested, of course). I keep my imaginary skillet with me at all times. It comes in handy to imagine using it on a few faces. (My sister has the imaginary baseball bat for a few select individuals.)
I would love to be his boss, but I can't see it happening as the little tur4 is already top of his tree and I am the wrong nationality for where I work, I just hope that the branches are deep in his never regions giving him some irritation...
I need something a little more immediate (and anonymous)