Take a screen capture of someone's screen. It would work best if they have a program open. Then save the screen capture as an image and use that as their desktop background picture. It will confuse the heck out of them when they see their desktop icons over their microsoft word.

Without disguising your voice, compliment yourself over the office intercom. Compliment yourself on a just completed task or give yourself the rest of the day off.

Relabel the data CDs so they say 'Porn' on them.

The next time someone at the office wins an award, send out a memo to everyone that the award has been discontinued.

Place some clear tape over the calculator to make the numbers very fuzzy.

Change the speed dial labels on office phones. It will piss off your co-worker when he is trying to call tech support but instead speed dials the courier.

Take something that your co-worker needs. Such as his mouse and leave a ransom note in its place. Don't tell him who you are. Just tell him where to leave the money for it.

Ask your victim if he heard about the rumor going around, tell him people are saying he is getting fire today. It helps if multiple co-worker also tell him the same thing. So plan ahead and get others in on the prank.

Turn your victim's speaker volume up full blast. He won't know until the next time his computer makes a sound, it will surprise him to death! haha

Put a small animal in the file cabinet. Make sure its a cabinet that is opened multiple times daily.

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"

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.

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

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.

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