Pop two “minor” keys off his keyboard and swap them. Ones he won’t notice are swapped right away (like B and V). If he’s a touch typist, he won’t notice the keys are swapped and think he’s just making unusual typos.
Alternatively, remove the hinges from his office door and reattach them on the other side.
Your coworkers are a pack of immature idiots with no sense of judgement. People have been seriously injured because somebody thought it would be “funny” to tamper with their food.
If you REALLY want this crap to stop, compose an e-mail describing your workplace bullying experiences and send it to your boss. That way you’ll start a paper trail. If your boss fails to address the situation, then go to Human Resources.
Who the hell do you work with? Johnny Knoxville and Bam Margera?
If you do anything prank-wise in retaliation, it will automatically invalidate any future complaints to the management.
Get the ghost peppers and rub it on their mouse and keyboard
You could just ctrl-alt-arrow key to screw up their monitor.
Find one of those “canned air” computer duster offer things hanging around the office. Turn it upside down so it will spray whatever ozone destroying chemical that is in there in liquid form. Spray a nice quantity of that liquid into something like a tupperware container with a tight but popable lid. Close said lid and quickly chunk it into their office/cubicle.
Do a test run first to see how bad the boom will be so you don’t end up in Gitmo.
Most of these pranks sound pretty douchey and unfunny. And in most workplaces, they would probably be pretty close to, if not actual grounds for dismissal.
That said, here’s an actual prank I played on a coworker. I found an online text to speech converter:
http://www.oddcast.com/home/demos/tts/tts_example.php
…and as it happens, one of the voices sounds exactly like our biggest, most demanding client. I’ll leave the rest to your imagination.
Superglue everything.
Can you grind their parents up and then have a chili cookoff?
Chessic Sense, most people seem to be suggesting going to HR. I think we need to know how you actually feel about these pranks. Is this something that you’re currently laughing off or is this something you’re pissed off about? Because if you’re pissed off, then you need at LEAST tell them to stop, right now. And then go to HR.
But if this is all just in fun and games, well, then everyone else needs to lighten up.
A standard soft drink cup, such as used at fast food places, can be carefully sliced all the way around (about 1/4" from the bottom) with a sharp knife and still retain its contents. Until someone lifts the cup.
The Thinkgeek Eviltron.
While reviewing your wonderful suggestions, I’ve decided to bide my time by jamming rolled-up paper into their chair elevator latch, holding it permanently and inconspicuously open. Now when they sit down, the chair sinks to the floor, but when they get up, it pops back up again. I’ve even bugged my own chair, so that when they inevitably switch chairs with mine while I’m gone, I’ll get them each twice or even three times.
If this continues, I’ll loosely tie some fishing line to the wheels of their chair and their desk items, so that when they push their chair into my cubicle, all their stuff ends up on the floor.
I’m currently waiting for the right time to do the ol’ “upside down cup of water” one where you seal a full coffee cup with card stock or something, flip it over onto the desk and remove the card, table-cloth-style.
Some of the pranks are funny, like when they saran wrapped everything on my desk, including pictures in the picture frames and each individual pen, pencil, and highlighter. Others, like the peppers, cross the line. We had a clearly established “no food or drink” rule, but they violated that and now must pay.
They’re bored, and they’re too afraid of each other to target each other.
Saran wrap and tape their cubicle door (or office door) closed, then fill the cubicle/office with packing peanuts.
We once moved a co-worker’s desk, file cabinet, computer, phone, name tag - everything, into a storage closet in the men’s room. We had to engage IT and facilities for this. When he got back from vacation we all had a good laugh, as did he.
This. +100 on it.
It is one of the most evil, insidious devices ever invented. I got primo payback on a cow-orker with one; almost reduced him to chewing on his cubicle. 
Or wait until they go to use the urinal and …
The ghost pepper was all kinds of fucked up. And it wasn’t a sliver of one but three?! For my, that is not a prank but a physical assault and I would treat it as such.