Old Toy
Use an old toy that has a button which is used to activate sound. Such as a baby crying. Then tape it to the back of the victims desk drawer, but don't close it completely, leave it open just a crack. When your victim decides to close it, he will hear the annoying sound and wonder where its coming from.

Something is wrong with the copier!
Leave a copier to print 99 copies at 33% resolution on 8x14 paper.

Upside Down Cup
Fill a cup with water about half way. Then place an index card over the cup. Then turn the cup upside down on your co-worker's desk. Finally, carefully slide the index card out from underneath the cup. Whenever your co-worker decides to pick the cup up, he will be drenched in water.

Auto Correct
Microsoft Word has a feature called 'Auto Correct'. It will auto correct any mispelling such as 'TEH' which will automatically be changed to 'THE'. Anyone can go into the tool and change the words. When your co-worker is away from his desk. Change a few of his common words around.

Hey! I wanted snacks!
Empty boxes of common snacks such as Cheez-Its or Figs and replace them with shredded paper or anything else that will weigh the box down. Then place them in the break room.

Bumper Sticker Prank
Make your own bumper sticker with tape and paper and put it on your co-workers car. Get creative with the message.

Did I Really Park There?
If you have one of those co-workers that parks in the same spot every day, then try this prank. When he isn't paying attention, take his keys and move his car over just one space.

Party Mix
At the next pot luck luncheon. You bring the Chex Party Mix. But get creative, mix in some Pup-eroni dog food snack. Just break them into little pieces and throw them in the same bowl...stir it up and nobody will tell the difference

Format C:
While your co-worker next to you takes a little break, type "Format C:" on his computer screen. Then disconnect his keyboard and mouse. Sit back and wait til he returns.

Glass Cleaner In The Water Cooler
Ok, make sure you don't actually pour any glass cleaner in the water cooler...this is dangerous and very poisonous. A better prank would be to just pour a few drops of blue food coloring in the water and just place a sign on the bottle that reads, "Glass Cleaner"